diff --git a/src/ifcpatch/ifcpatch/recipes/DowngradeIndexedPolyCurve.py b/src/ifcpatch/ifcpatch/recipes/DowngradeIndexedPolyCurve.py
index 4ae36f26b8..259fc7aa18 100644
--- a/src/ifcpatch/ifcpatch/recipes/DowngradeIndexedPolyCurve.py
+++ b/src/ifcpatch/ifcpatch/recipes/DowngradeIndexedPolyCurve.py
@@ -17,6 +17,12 @@
 # along with IfcPatch.  If not, see <http://www.gnu.org/licenses/>.
 
 import ifcopenshell.util.element
+import ifcopenshell.util.shape_builder
+
+# Number of straight chords used to approximate one IfcArcIndex when flattening
+# an IfcIndexedPolyCurve to an IfcPolyline. Higher values track the true arc
+# more closely at the cost of file weight.
+ARC_SUBDIVISION = 16
 
 
 class Patcher:
@@ -34,6 +40,9 @@ class Patcher:
         an IFC4 model (IFC2X3 does not have this geometry type) to help
         compatibility in viewers like Navisworks.
 
+        Arc segments (``IfcArcIndex``) are approximated by a chord polyline
+        through ``ARC_SUBDIVISION`` evenly-spaced points along the arc.
+
         Example:
 
             ifcpatch.execute({"input": "input.ifc", "file": model, "recipe": "DowngradeIndexedPolyCurve", "arguments": []})
@@ -47,19 +56,46 @@ class Patcher:
         curve_map = {}
 
         for curve in self.file.by_type("IfcIndexedPolyCurve"):
-            if "IfcArcIndex" in [s.is_a() for s in curve.Segments]:
-                print("Could not convert curve due to arcs", curve)
-                continue
             coordinates = curve.Points.CoordList
-            points = []
-            for i, segment in enumerate(curve.Segments):
-                segment = segment.wrappedValue
-                if i == 0:
-                    points.append(self.file.createIfcCartesianPoint(coordinates[segment[0] - 1]))
-                points.append(self.file.createIfcCartesianPoint(coordinates[segment[1] - 1]))
-            polyline = self.file.create_entity("IfcPolyline", points)
+            segments = curve.Segments
+            if segments is None:
+                # IFC4: an absent Segments list means the curve is a polyline
+                # through every CoordList point in declared order.
+                points = [tuple(c) for c in coordinates]
+            else:
+                points = self._segments_to_points(segments, coordinates)
+                if points is None:
+                    continue
+            ifc_points = [self.file.createIfcCartesianPoint(p) for p in points]
+            polyline = self.file.create_entity("IfcPolyline", ifc_points)
             curve_map[curve] = polyline
 
         for curve, polyline in curve_map.items():
-            for inverse in self.file.get_inverse(curve):
-                ifcopenshell.util.element.replace_attribute(inverse, curve, polyline)
+            ifcopenshell.util.element.replace_element(curve, polyline)
+
+    def _segments_to_points(self, segments, coordinates):
+        points: list[tuple[float, ...]] = []
+        for i, segment in enumerate(segments):
+            indices = segment.wrappedValue
+            if segment.is_a("IfcArcIndex"):
+                if len(indices) != 3:
+                    return None
+                arc_points = ifcopenshell.util.shape_builder.arc_to_polyline_points(
+                    coordinates[indices[0] - 1],
+                    coordinates[indices[1] - 1],
+                    coordinates[indices[2] - 1],
+                    ARC_SUBDIVISION,
+                )
+                if i == 0:
+                    points.append(tuple(arc_points[0]))
+                points.extend(tuple(p) for p in arc_points[1:])
+            else:
+                # IfcLineIndex is LIST [2:?] OF IfcPositiveInteger — a polyline
+                # through every listed index. Skip the first index on non-leading
+                # segments since it duplicates the previous segment's endpoint.
+                seg_points = [tuple(coordinates[idx - 1]) for idx in indices]
+                if i == 0:
+                    points.extend(seg_points)
+                else:
+                    points.extend(seg_points[1:])
+        return points
diff --git a/src/ifcpatch/ifcpatch/recipes/Migrate.py b/src/ifcpatch/ifcpatch/recipes/Migrate.py
index 627342f096..c7479a6121 100644
--- a/src/ifcpatch/ifcpatch/recipes/Migrate.py
+++ b/src/ifcpatch/ifcpatch/recipes/Migrate.py
@@ -20,7 +20,9 @@ from logging import Logger
 from typing import Union
 
 import ifcopenshell
+import ifcopenshell.util.element
 import ifcopenshell.util.schema
+import ifcopenshell.util.shape_builder
 
 import ifcpatch
 
@@ -32,10 +34,39 @@ class Patcher(ifcpatch.BasePatcher):
         logger: Union[Logger, None] = None,
         schema: ifcopenshell.util.schema.IFC_SCHEMA = "IFC4",
     ):
-        """Migrate from one IFC version to another
+        """Migrate from one IFC version to another.
 
-        Note that this is experimental and will try to preserve as much data as
-        possible. Upgrading to IFC4 is more stable than downgrading to IFC2X3.
+        The recipe iterates every entity in the source file and rewrites it
+        into a new file with the target schema, delegating per-entity class /
+        attribute translation to :class:`ifcopenshell.util.schema.Migrator`.
+        Upgrades (IFC2X3 → IFC4, IFC4 → IFC4X3) are best supported because the
+        target schema is a superset; downgrades are lossy by definition (see
+        below). Entities that fail to migrate are collected; on completion a
+        summary ``RuntimeError`` is raised listing up to 20 failures.
+
+        IFC4 → IFC2X3 downgrade additionally runs a preprocessing pipeline so
+        IFC4-only geometry and element classes survive the schema gap:
+
+        - ``IfcIndexedPolyCurve`` (including arc segments, approximated by a
+          chord polyline) is flattened to ``IfcPolyline``.
+        - ``IfcPolygonalFaceSet`` and ``IfcTriangulatedFaceSet`` are converted
+          directly to ``IfcFacetedBrep`` at the entity level, preserving the
+          original mesh topology.
+        - Orphan IFC4-only geometry instances left over after the rewires are
+          purged so the migration loop does not trip on them.
+        - IFC4-only ``IfcElement`` subclasses (``IfcLamp``, ``IfcPipeSegment``,
+          ``IfcGeographicElement``, …) fall back to ``IfcBuildingElementProxy``
+          via the Migrator's ``fallback_element_to_proxy`` opt-in. The
+          original class and ``PredefinedType`` are encoded into
+          ``ObjectType`` (e.g. ``"IfcLamp/COMPACTFLUORESCENT"``) when
+          ``ObjectType`` is empty, so the type information survives the
+          downgrade.
+
+        Non-element IFC4-only entities (relationships, geometry items outside
+        any product, …) that have no direct equivalent still raise
+        ``NotImplementedError`` from the Migrator with the failing class and
+        inverse references named, instead of the cryptic
+        ``Entity with name '' not found in schema 'IFC2X3'``.
 
         :param schema: The schema identifier of the IFC version to migrate to.
 
@@ -50,10 +81,105 @@ class Patcher(ifcpatch.BasePatcher):
         self.schema = schema
 
     def patch(self):
+        # IFC4 and IFC4X3 both have geometry / element classes absent in
+        # IFC2X3, so both source schemas need the downgrade preprocessing +
+        # IfcBuildingElementProxy fallback when targeting IFC2X3.
+        is_downgrade_to_ifc2x3 = self.schema == "IFC2X3" and self.file.schema in ("IFC4", "IFC4X3")
+        if is_downgrade_to_ifc2x3:
+            self._prepare_for_downgrade()
+
         self.file_patched = ifcopenshell.file(schema=self.schema)
-        migrator = ifcopenshell.util.schema.Migrator()
+        migrator = ifcopenshell.util.schema.Migrator(fallback_element_to_proxy=is_downgrade_to_ifc2x3)
         migrator.preprocess(self.file, self.file_patched)
+
+        migrated = 0
+        failures: list[tuple[ifcopenshell.entity_instance, Exception]] = []
         for element in self.file:
-            new_element = migrator.migrate(element, self.file_patched)
-            print("Migrating", element)
-            print("Successfully converted to", new_element)
+            try:
+                migrator.migrate(element, self.file_patched)
+                migrated += 1
+            except Exception as exc:
+                failures.append((element, exc))
+
+        if is_downgrade_to_ifc2x3:
+            self._encode_fallback_class_into_object_type(migrator)
+
+        # BasePatcher.__init__ guarantees self.logger is non-None
+        # (ensure_logger falls back to logging.getLogger("IFCPatch")).
+        self.logger.info(f"Migrated {migrated} entities to {self.schema}.")
+        if failures:
+            summary = [f"{len(failures)} entities could not be migrated to {self.schema}:"]
+            for element, exc in failures[:20]:
+                summary.append(f"  #{element.id()}={element.is_a()}: {exc}")
+            if len(failures) > 20:
+                summary.append(f"  … (+{len(failures) - 20} more)")
+            raise RuntimeError("\n".join(summary))
+
+    def _prepare_for_downgrade(self) -> None:
+        from ifcpatch.recipes.DowngradeIndexedPolyCurve import Patcher as DowngradePolyCurve
+
+        DowngradePolyCurve(self.file, self.logger).patch()
+        self._convert_face_sets_to_faceted_brep()
+        self._purge_orphaned_ifc4_only_entities()
+
+    def _convert_face_sets_to_faceted_brep(self) -> None:
+        face_sets = list(self.file.by_type("IfcPolygonalFaceSet")) + list(self.file.by_type("IfcTriangulatedFaceSet"))
+        if not face_sets:
+            return
+
+        # IfcShapeRepresentations carrying these face sets need their type tag
+        # updated from "Tessellation" (IFC4) to "Brep" (IFC2X3-compatible).
+        # Snapshot the relevant inverses before rewiring — the inverse set is
+        # invalidated once replace_element runs.
+        touched_reps: set[int] = set()
+        for face_set in face_sets:
+            faceted_brep = ifcopenshell.util.shape_builder.polygonal_face_set_to_faceted_brep(face_set)
+            touched_reps.update(
+                inv.id() for inv in self.file.get_inverse(face_set) if inv.is_a("IfcShapeRepresentation")
+            )
+            ifcopenshell.util.element.replace_element(face_set, faceted_brep)
+
+        for rep_id in touched_reps:
+            self.file.by_id(rep_id).RepresentationType = "Brep"
+
+    def _purge_orphaned_ifc4_only_entities(self) -> None:
+        # Preprocessing rewires references away from source-schema-only
+        # carriers but does not delete the now-unreferenced instances
+        # themselves. Sweep iteratively so cascades collapse leaf-first
+        # (curves → point lists, face sets → indexed faces → point lists).
+        # Scoped to the actual source schema so IFC4X3 → IFC2X3 downgrades
+        # also catch IFC4X3-only geometry (IfcAlignmentCurve etc.), not just
+        # the IFC4 gap.
+        targets = ifcopenshell.util.schema.geometry_classes_introduced_after(
+            self.schema, source_schema=self.file.schema
+        )
+        while True:
+            removed = False
+            for ifc_class in targets:
+                for entity in list(self.file.by_type(ifc_class)):
+                    if not self.file.get_inverse(entity):
+                        self.file.remove(entity)
+                        removed = True
+            if not removed:
+                break
+
+    def _encode_fallback_class_into_object_type(self, migrator: ifcopenshell.util.schema.Migrator) -> None:
+        # IFC4-only IfcElement subclasses (IfcLamp, IfcPipeSegment, …) migrate
+        # as IfcBuildingElementProxy. The subclass identity + its PredefinedType
+        # would otherwise be silently lost — IFC2X3 IfcBuildingElementProxy has
+        # no slot for them. Encode "<OriginalClass>/<PredefinedType>" into
+        # ObjectType when empty (don't trample author-supplied values).
+        for source_id, new_id in migrator.migrated_ids.items():
+            try:
+                source = self.file.by_id(source_id)
+                new = self.file_patched.by_id(new_id)
+            except RuntimeError:
+                continue
+            if not new.is_a("IfcBuildingElementProxy"):
+                continue
+            if source.is_a("IfcBuildingElementProxy"):
+                continue
+            if getattr(new, "ObjectType", None):
+                continue
+            predef = getattr(source, "PredefinedType", None)
+            new.ObjectType = f"{source.is_a()}/{predef}" if predef else source.is_a()
diff --git a/src/ifcpatch/test/test_DowngradeIndexedPolyCurve.py b/src/ifcpatch/test/test_DowngradeIndexedPolyCurve.py
new file mode 100644
index 0000000000..adeda0b669
--- /dev/null
+++ b/src/ifcpatch/test/test_DowngradeIndexedPolyCurve.py
@@ -0,0 +1,137 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2026 Bonsai Contributors
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+import ifcpatch
+import test.bootstrap
+
+
+class TestDowngradeIndexedPolyCurve(test.bootstrap.IFC4):
+    def _make_curve(self, segments=None):
+        point_list = self.file.create_entity(
+            "IfcCartesianPointList2D",
+            CoordList=[(0.0, 0.0), (1.0, 0.0), (1.0, 1.0)],
+        )
+        curve = self.file.create_entity(
+            "IfcIndexedPolyCurve",
+            Points=point_list,
+            Segments=segments,
+        )
+        self.file.create_entity(
+            "IfcArbitraryClosedProfileDef", ProfileType="AREA", OuterCurve=curve
+        )
+        return curve
+
+    def test_run_without_segments(self):
+        """An IfcIndexedPolyCurve with no Segments must downgrade to an
+        IfcPolyline through every CoordList point in order — IFC4 defines
+        the implicit-polyline meaning of an absent Segments list, and the
+        ifcopenshell shape builder emits this form for simple open curves."""
+        self._make_curve(segments=None)
+        ifcpatch.execute(
+            {"input": "input.ifc", "file": self.file, "recipe": "DowngradeIndexedPolyCurve", "arguments": []}
+        )
+        polylines = self.file.by_type("IfcPolyline")
+        assert len(polylines) == 1
+        assert len(polylines[0].Points) == 3
+
+    def test_run_with_line_segments(self):
+        """Line-segmented IfcIndexedPolyCurves downgrade to an equivalent IfcPolyline."""
+        segments = [
+            self.file.createIfcLineIndex((1, 2)),
+            self.file.createIfcLineIndex((2, 3)),
+        ]
+        self._make_curve(segments=segments)
+        ifcpatch.execute(
+            {"input": "input.ifc", "file": self.file, "recipe": "DowngradeIndexedPolyCurve", "arguments": []}
+        )
+        polylines = self.file.by_type("IfcPolyline")
+        assert len(polylines) == 1
+        assert len(polylines[0].Points) == 3
+
+    def test_run_with_multi_index_line_segment(self):
+        """An IfcLineIndex with >2 indices encodes a polyline through every
+        index — the downgraded IfcPolyline must include every one of them.
+        This is the canonical form Bonsai's shape builder emits for closed
+        rectangle profiles (e.g. parametric wall body outlines), serialised
+        as ``IfcIndexedPolyCurve(Points, (IfcLineIndex((1,2,3,4,1))))``."""
+        point_list = self.file.create_entity(
+            "IfcCartesianPointList2D",
+            CoordList=[(0.0, 0.0), (1.0, 0.0), (1.0, 1.0), (0.0, 1.0)],
+        )
+        curve = self.file.create_entity(
+            "IfcIndexedPolyCurve",
+            Points=point_list,
+            Segments=[self.file.createIfcLineIndex((1, 2, 3, 4, 1))],
+        )
+        self.file.create_entity(
+            "IfcArbitraryClosedProfileDef", ProfileType="AREA", OuterCurve=curve
+        )
+        ifcpatch.execute(
+            {"input": "input.ifc", "file": self.file, "recipe": "DowngradeIndexedPolyCurve", "arguments": []}
+        )
+        polylines = self.file.by_type("IfcPolyline")
+        assert len(polylines) == 1
+        assert len(polylines[0].Points) == 5
+        coords = [p.Coordinates for p in polylines[0].Points]
+        assert coords[0] == coords[-1] == (0.0, 0.0)
+        assert coords[1] == (1.0, 0.0)
+        assert coords[2] == (1.0, 1.0)
+        assert coords[3] == (0.0, 1.0)
+
+    def test_run_with_chained_multi_index_segments(self):
+        """When two IfcLineIndex segments are chained, the shared endpoint
+        between them must appear once, not twice."""
+        point_list = self.file.create_entity(
+            "IfcCartesianPointList2D",
+            CoordList=[(0.0, 0.0), (1.0, 0.0), (1.0, 1.0), (0.0, 1.0)],
+        )
+        curve = self.file.create_entity(
+            "IfcIndexedPolyCurve",
+            Points=point_list,
+            Segments=[
+                self.file.createIfcLineIndex((1, 2, 3)),
+                self.file.createIfcLineIndex((3, 4)),
+            ],
+        )
+        self.file.create_entity(
+            "IfcArbitraryClosedProfileDef", ProfileType="AREA", OuterCurve=curve
+        )
+        ifcpatch.execute(
+            {"input": "input.ifc", "file": self.file, "recipe": "DowngradeIndexedPolyCurve", "arguments": []}
+        )
+        polylines = self.file.by_type("IfcPolyline")
+        assert len(polylines) == 1
+        coords = [p.Coordinates for p in polylines[0].Points]
+        assert coords == [(0.0, 0.0), (1.0, 0.0), (1.0, 1.0), (0.0, 1.0)]
+
+    def test_run_facets_arc_segments(self):
+        """Arc-segmented IfcIndexedPolyCurves are downgraded by sampling the
+        circular arc into a chord polyline. The chord count is fixed by
+        the recipe's subdivision parameter."""
+        from ifcpatch.recipes.DowngradeIndexedPolyCurve import ARC_SUBDIVISION
+
+        segments = [self.file.createIfcArcIndex((1, 2, 3))]
+        self._make_curve(segments=segments)
+        ifcpatch.execute(
+            {"input": "input.ifc", "file": self.file, "recipe": "DowngradeIndexedPolyCurve", "arguments": []}
+        )
+        polylines = self.file.by_type("IfcPolyline")
+        assert len(polylines) == 1
+        assert len(polylines[0].Points) == ARC_SUBDIVISION + 1
diff --git a/src/ifcpatch/test/test_Migrate.py b/src/ifcpatch/test/test_Migrate.py
index 85e60ff09f..23930e6283 100644
--- a/src/ifcpatch/test/test_Migrate.py
+++ b/src/ifcpatch/test/test_Migrate.py
@@ -17,6 +17,9 @@
 # along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
 
 
+import pytest
+
+import ifcopenshell.api.project
 import ifcpatch
 import test.bootstrap
 
@@ -27,3 +30,181 @@ class TestMigrate(test.bootstrap.IFC4):
         old_file.header.file_name.name = "test"
         new_file = ifcpatch.execute({"file": old_file, "recipe": "Migrate", "arguments": ["IFC4"]})
         assert new_file.header.file_name.name == "test"
+
+    def test_migrate_ifc4_to_ifc2x3_flattens_indexed_polycurve(self):
+        """Downgrade IFC4 → IFC2X3 should auto-run DowngradeIndexedPolyCurve on
+        IfcIndexedPolyCurve carriers, so the migrated file uses IfcPolyline (which
+        exists in IFC2X3) instead of crashing on the IFC4-only curve class."""
+        ifc4_file = self.file
+        point_list = ifc4_file.create_entity(
+            "IfcCartesianPointList2D",
+            CoordList=((0.0, 0.0), (1.0, 0.0), (1.0, 1.0), (0.0, 1.0)),
+        )
+        segments = [
+            ifc4_file.create_entity("IfcLineIndex", (1, 2)),
+            ifc4_file.create_entity("IfcLineIndex", (2, 3)),
+            ifc4_file.create_entity("IfcLineIndex", (3, 4)),
+            ifc4_file.create_entity("IfcLineIndex", (4, 1)),
+        ]
+        curve = ifc4_file.create_entity(
+            "IfcIndexedPolyCurve", Points=point_list, Segments=segments, SelfIntersect=False
+        )
+        ifc4_file.create_entity("IfcArbitraryClosedProfileDef", ProfileType="AREA", OuterCurve=curve)
+
+        new_file = ifcpatch.execute({"file": ifc4_file, "recipe": "Migrate", "arguments": ["IFC2X3"]})
+
+        assert new_file.schema == "IFC2X3"
+        new_profile = new_file.by_type("IfcArbitraryClosedProfileDef")[0]
+        assert new_profile.OuterCurve.is_a("IfcPolyline")
+        # The preprocessing step should have purged orphaned IFC4-only entities
+        # from the source before the migration loop reached them.
+        assert not ifc4_file.by_type("IfcIndexedPolyCurve")
+        assert not ifc4_file.by_type("IfcCartesianPointList2D")
+
+    def test_migrate_ifc4_to_ifc2x3_encodes_fallback_class_in_object_type(self):
+        """IfcLamp / IfcPipeSegment / IfcGeographicElement fall back to
+        IfcBuildingElementProxy on downgrade. The original class and
+        PredefinedType are encoded into ObjectType so the type info survives
+        — but only when ObjectType is empty (author-supplied values stay)."""
+        ifc4_file = self.file
+        ifc4_file.create_entity("IfcLamp", GlobalId="2K6Z3DR8X37AS9XFvX8GcW", PredefinedType="COMPACTFLUORESCENT")
+        ifc4_file.create_entity("IfcPipeSegment", GlobalId="0_bkftCTnBCOOZeUxtJngE")
+        ifc4_file.create_entity(
+            "IfcGeographicElement",
+            GlobalId="3_b4gD1aP3ARmIm2ePijXi",
+            ObjectType="Terrain Mesh",  # author-supplied, must not be overwritten
+            PredefinedType="TERRAIN",
+        )
+
+        new_file = ifcpatch.execute({"file": ifc4_file, "recipe": "Migrate", "arguments": ["IFC2X3"]})
+
+        proxies = {p.GlobalId: p for p in new_file.by_type("IfcBuildingElementProxy")}
+        # IfcLamp with no author ObjectType: encoded as IfcLamp/COMPACTFLUORESCENT.
+        assert proxies["2K6Z3DR8X37AS9XFvX8GcW"].ObjectType == "IfcLamp/COMPACTFLUORESCENT"
+        # IfcPipeSegment with no PredefinedType set: just the class name.
+        assert proxies["0_bkftCTnBCOOZeUxtJngE"].ObjectType == "IfcPipeSegment"
+        # IfcGeographicElement with author ObjectType: preserved as-is.
+        assert proxies["3_b4gD1aP3ARmIm2ePijXi"].ObjectType == "Terrain Mesh"
+
+    def test_migrate_ifc4_to_ifc2x3_converts_polygonal_face_set_to_faceted_brep(self):
+        """IfcPolygonalFaceSet has no IFC2X3 equivalent. Direct entity-level
+        conversion produces an IfcFacetedBrep with the same topology, regardless
+        of which representation context the source lived in."""
+        ifc4_file = self.file
+        coords = ifc4_file.create_entity(
+            "IfcCartesianPointList3D",
+            CoordList=(
+                (0.0, 0.0, 0.0),
+                (1.0, 0.0, 0.0),
+                (1.0, 1.0, 0.0),
+                (0.0, 1.0, 0.0),
+                (0.5, 0.5, 1.0),
+            ),
+        )
+        # Square base + 4 triangle sides — a simple pyramid.
+        faces = [
+            ifc4_file.create_entity("IfcIndexedPolygonalFace", CoordIndex=(1, 2, 3, 4)),
+            ifc4_file.create_entity("IfcIndexedPolygonalFace", CoordIndex=(1, 2, 5)),
+            ifc4_file.create_entity("IfcIndexedPolygonalFace", CoordIndex=(2, 3, 5)),
+            ifc4_file.create_entity("IfcIndexedPolygonalFace", CoordIndex=(3, 4, 5)),
+            ifc4_file.create_entity("IfcIndexedPolygonalFace", CoordIndex=(4, 1, 5)),
+        ]
+        face_set = ifc4_file.create_entity("IfcPolygonalFaceSet", Coordinates=coords, Faces=faces)
+        context = ifc4_file.create_entity(
+            "IfcGeometricRepresentationContext",
+            ContextType="Model",
+            CoordinateSpaceDimension=3,
+            Precision=0.01,
+            WorldCoordinateSystem=ifc4_file.createIfcAxis2Placement3D(
+                Location=ifc4_file.createIfcCartesianPoint((0.0, 0.0, 0.0))
+            ),
+        )
+        ifc4_file.create_entity(
+            "IfcShapeRepresentation",
+            ContextOfItems=context,
+            RepresentationIdentifier="Body",
+            RepresentationType="Tessellation",
+            Items=[face_set],
+        )
+
+        new_file = ifcpatch.execute({"file": ifc4_file, "recipe": "Migrate", "arguments": ["IFC2X3"]})
+
+        assert new_file.schema == "IFC2X3"
+        breps = new_file.by_type("IfcFacetedBrep")
+        assert len(breps) == 1
+        brep = breps[0]
+        assert len(brep.Outer.CfsFaces) == 5
+        # Coordinates from the source CartesianPointList3D must appear in the
+        # resulting brep's loop points — otherwise the conversion silently
+        # corrupted geometry.
+        brep_coords = {tuple(p.Coordinates) for face in brep.Outer.CfsFaces for p in face.Bounds[0].Bound.Polygon}
+        for expected in ((0.0, 0.0, 0.0), (1.0, 0.0, 0.0), (1.0, 1.0, 0.0), (0.0, 1.0, 0.0), (0.5, 0.5, 1.0)):
+            assert expected in brep_coords, f"vertex {expected} missing from converted brep"
+        rep = new_file.by_type("IfcShapeRepresentation")[0]
+        assert rep.RepresentationType == "Brep"
+        assert rep.Items[0].is_a("IfcFacetedBrep")
+
+    def test_migrate_ifc4_to_ifc2x3_summarises_unmappable_entities(self):
+        """When an IFC4-only entity that cannot be auto-substituted survives
+        preprocessing, the recipe must surface a summary RuntimeError naming
+        the failing class — not the cryptic ``RuntimeError: Entity with name
+        '' not found``.
+
+        Uses ``IfcWorkCalendar`` as the fixture — an IFC4 entity that
+        (a) is not an IfcRepresentationItem (skips the geometry purge),
+        (b) is not an IfcElement (skips the proxy fallback),
+        (c) has no IFC2X3 equivalent in ``class_4_to_2x3.json`` (mapped to ``""``).
+        These three conditions together guarantee it always reaches the
+        unmappable error path, independent of future schema additions."""
+        ifc4_file = self.file
+        ifc4_file.create_entity("IfcWorkCalendar", GlobalId="2K6Z3DR8X37AS9XFvX8GcW")
+
+        with pytest.raises(RuntimeError) as exc_info:
+            ifcpatch.execute({"file": ifc4_file, "recipe": "Migrate", "arguments": ["IFC2X3"]})
+
+        message = str(exc_info.value)
+        assert "IfcWorkCalendar" in message
+
+    def test_migrate_ifc4x3_to_ifc2x3_runs_downgrade_preprocessing(self):
+        """IFC4X3 → IFC2X3 must trigger the same downgrade preprocessing as
+        IFC4 → IFC2X3: curve flatten, face-set → brep, IfcBuildingElementProxy
+        fallback, ObjectType encoding. Pins the gate at
+        ``self.file.schema in ('IFC4', 'IFC4X3')`` — a narrower check would
+        silently leave IFC4X3 sources crashing on IFC4-only geometry."""
+        ifc4x3_file = ifcopenshell.api.project.create_file(version="IFC4X3")
+        ifc4x3_file.create_entity("IfcLamp", GlobalId="2K6Z3DR8X37AS9XFvX8GcW", PredefinedType="COMPACTFLUORESCENT")
+
+        new_file = ifcpatch.execute({"file": ifc4x3_file, "recipe": "Migrate", "arguments": ["IFC2X3"]})
+
+        assert new_file.schema == "IFC2X3"
+        proxies = new_file.by_type("IfcBuildingElementProxy")
+        assert len(proxies) == 1
+        # ObjectType encoding ran — same as the IFC4 → IFC2X3 case.
+        assert proxies[0].ObjectType == "IfcLamp/COMPACTFLUORESCENT"
+
+    def test_migrate_ifc4_to_ifc2x3_flattens_arc_bearing_indexed_polycurve(self):
+        """An IfcIndexedPolyCurve with IfcArcIndex segments is approximated
+        with a chord polyline rather than skipped, so the parent profile def
+        and its representations stay parametric (no fallback to tessellation)."""
+        ifc4_file = self.file
+        point_list = ifc4_file.create_entity(
+            "IfcCartesianPointList2D",
+            CoordList=((1.0, 0.0), (0.0, 1.0), (-1.0, 0.0), (0.0, -1.0)),
+        )
+        # Two half-arcs forming a circle: (1,0)→(0,1)→(-1,0)→(0,-1)→(1,0).
+        segments = [
+            ifc4_file.create_entity("IfcArcIndex", (1, 2, 3)),
+            ifc4_file.create_entity("IfcArcIndex", (3, 4, 1)),
+        ]
+        curve = ifc4_file.create_entity(
+            "IfcIndexedPolyCurve", Points=point_list, Segments=segments, SelfIntersect=False
+        )
+        ifc4_file.create_entity("IfcArbitraryClosedProfileDef", ProfileType="AREA", OuterCurve=curve)
+
+        new_file = ifcpatch.execute({"file": ifc4_file, "recipe": "Migrate", "arguments": ["IFC2X3"]})
+
+        assert new_file.schema == "IFC2X3"
+        new_profile = new_file.by_type("IfcArbitraryClosedProfileDef")[0]
+        assert new_profile.OuterCurve.is_a("IfcPolyline")
+        # Arc subdivision should produce many more points than the 4 input coords.
+        assert len(new_profile.OuterCurve.Points) > 4