diff --git a/src/bonsai/bonsai/bim/module/root/operator.py b/src/bonsai/bonsai/bim/module/root/operator.py
index ca84543dea..62586e8c30 100644
--- a/src/bonsai/bonsai/bim/module/root/operator.py
+++ b/src/bonsai/bonsai/bim/module/root/operator.py
@@ -77,6 +77,7 @@ class DisableReassignClass(bpy.types.Operator):
 class ReassignClass(bpy.types.Operator):
     bl_idname = "bim.reassign_class"
     bl_label = "Reassign IFC Class"
+    bl_description = "Reassign IFC class for selected objects"
     bl_options = {"REGISTER", "UNDO"}
     obj: bpy.props.StringProperty()
 
@@ -89,71 +90,71 @@ class ReassignClass(bpy.types.Operator):
         else:
             objects = set(context.selected_objects + [context.active_object])
         self.file = IfcStore.get_file()
-        predefined_type = context.scene.BIMRootProperties.ifc_predefined_type
+        root_props = context.scene.BIMRootProperties
+        ifc_product: str = root_props.ifc_product
+        ifc_class: str = root_props.ifc_class
+        type_ifc_class = next(iter(ifcopenshell.util.type.get_applicable_types(ifc_class)), None)
+
+        predefined_type = root_props.ifc_predefined_type
         if predefined_type == "USERDEFINED":
-            predefined_type = context.scene.BIMRootProperties.ifc_userdefined_type
+            predefined_type = root_props.ifc_userdefined_type
 
         # NOTE: root.reassign_class
         # automatically will reassign class for other occurrences of the type
         # so we need to run it only for the types or non-typed elements
-        elements_to_reassign = set()
+        elements_to_reassign: dict[ifcopenshell.entity_instance, str] = dict()
         # need to update blender object name
         # for all elements that were changed in the process
-        elements_to_update = set()
+        elements_to_update: set[ifcopenshell.entity_instance] = set()
         for obj in objects:
-            obj.BIMObjectProperties.is_reassigning_class = False
             element = tool.Ifc.get_entity(obj)
-            if element.is_a("IfcTypeObject"):
-                elements_to_reassign.add(element)
-                elements_to_update.update(ifcopenshell.util.element.get_types(element))
+            if not element:
                 continue
 
-            # check if element is typed
-            element_type = ifcopenshell.util.element.get_type(element)
-            if element_type:
-                elements_to_reassign.add(element_type)
+            same_ifc_product = element.is_a(ifc_product)
+
+            if not same_ifc_product:
+                if not (element.is_a("IfcElement") and ifc_product == "IfcElementType") and not (
+                    element.is_a("IfcElementType") and ifc_product == "IfcElement"
+                ):
+                    self.report(
+                        {"ERROR"}, f"Not supported class reassignment for object '{obj.name}' -> {ifc_product}."
+                    )
+                    return {"CANCELLED"}
+
+            obj.BIMObjectProperties.is_reassigning_class = False
+            if element.is_a("IfcTypeObject"):
+                elements_to_reassign[element] = ifc_class
+                elements_to_update.update(ifcopenshell.util.element.get_types(element))
+                continue
+            elif same_ifc_product and (element_type := ifcopenshell.util.element.get_type(element)):
+                assert type_ifc_class
+                elements_to_reassign[element_type] = type_ifc_class
                 elements_to_update.update(ifcopenshell.util.element.get_types(element_type))
                 continue
 
             # non-typed element
-            elements_to_reassign.add(element)
+            elements_to_reassign[element] = ifc_class
 
         # store elements to objects to update later as elements will get invalid
         # after class reassignment
-        elements_to_update = elements_to_update | elements_to_reassign
+        elements_to_update = elements_to_update | set(elements_to_reassign)
         objects_to_update = set(o for e in elements_to_update if (o := tool.Ifc.get_object(e)))
 
-        base_class = context.scene.BIMRootProperties.ifc_class
-        if context.scene.BIMRootProperties.ifc_product == "IfcElementType":
-            type_class = base_class
-            occurrence_classes = ifcopenshell.util.type.get_applicable_entities(type_class)
-            occurrence_class = None if len(occurrence_classes) == 0 else occurrence_classes[0]
-        else:
-            occurrence_class = base_class
-            type_classes = ifcopenshell.util.type.get_applicable_types(occurrence_class)
-            type_class = None if len(type_classes) == 0 else type_classes[0]
-
         reassigned_elements = set()
-        for element in elements_to_reassign:
-            ifc_class = type_class if element.is_a("IfcTypeObject") else occurrence_class
-            if ifc_class is None:
-                self.report(
-                    {"ERROR"},
-                    f"Couldn't find valid class for reassigning element of class {element.is_a()} based on class {base_class}",
-                )
-                return {"CANCELLED"}
-
+        for element, ifc_class_ in elements_to_reassign.items():
             element = ifcopenshell.api.run(
                 "root.reassign_class",
                 self.file,
                 product=element,
-                ifc_class=ifc_class,
+                ifc_class=ifc_class_,
                 predefined_type=predefined_type,
             )
             reassigned_elements.add(element)
 
         for obj in objects_to_update:
             obj.name = tool.Loader.get_name(tool.Ifc.get_entity(obj))
+            tool.Collector.assign(obj)
         return {"FINISHED"}
 
 
diff --git a/src/bonsai/bonsai/bim/module/root/ui.py b/src/bonsai/bonsai/bim/module/root/ui.py
index 995d16cdcc..c8cec94d57 100644
--- a/src/bonsai/bonsai/bim/module/root/ui.py
+++ b/src/bonsai/bonsai/bim/module/root/ui.py
@@ -83,8 +83,7 @@ class BIM_PT_class(Panel):
     def draw_class_dropdowns(self, context, ifc_predefined_types, is_reassigning_class=False):
         props = context.scene.BIMRootProperties
         layout = self.layout
-        if not is_reassigning_class:
-            prop_with_search(layout, props, "ifc_product")
+        prop_with_search(layout, props, "ifc_product")
         prop_with_search(layout, props, "ifc_class")
         if ifc_predefined_types:
             prop_with_search(layout, props, "ifc_predefined_type")
diff --git a/src/bonsai/test/tool/test_root.py b/src/bonsai/test/tool/test_root.py
index 235ed3b04e..dda03e1ed3 100644
--- a/src/bonsai/test/tool/test_root.py
+++ b/src/bonsai/test/tool/test_root.py
@@ -173,7 +173,7 @@ class TestSetObjectName(NewFile):
 
 
 class TestReassignClass(NewFile):
-    def test_run(self):
+    def test_reassigning_multiple_occurrences_of_the_same_type(self):
         bpy.context.scene.BIMProjectProperties.template_file = "IFC4 Demo Template.ifc"
         bpy.ops.bim.create_project()
         ifc_file = tool.Ifc.get()
diff --git a/src/ifcopenshell-python/ifcopenshell/api/root/reassign_class.py b/src/ifcopenshell-python/ifcopenshell/api/root/reassign_class.py
index 3326f0b123..4eca742939 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/root/reassign_class.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/root/reassign_class.py
@@ -17,10 +17,16 @@
 # along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
 
 import ifcopenshell
+import ifcopenshell.api.aggregate
+import ifcopenshell.api.geometry
+import ifcopenshell.api.spatial
+import ifcopenshell.api.type
+import ifcopenshell.guid
+import ifcopenshell.util.representation
 import ifcopenshell.util.type
 import ifcopenshell.util.schema
 import ifcopenshell.util.element
-from typing import Optional
+from typing import Optional, Union, Literal
 
 
 def reassign_class(
@@ -46,15 +52,14 @@ def reassign_class(
     cannot be unambiguously derived, so you are required to manually check
     this.
 
+    Reassigning type class to occurrence (and vice versa) is supported.
+
     :param product: The IfcProduct that you want to change the class of.
     :type product: ifcopenshell.entity_instance
     :param ifc_class: The new IFC class you want to change it to.
-    :type ifc_class: str,optional
     :param predefined_type: In case you want to change the predefined type
         too. User defined types are also allowed, just type what you want.
-    :type predefined_type: str,optional
     :return: The newly modified product.
-    :rtype: ifcopenshell.entity_instance
 
     Example:
 
@@ -80,38 +85,141 @@ def reassign_class(
 
 
 class Usecase:
-    def execute(self):
-        element = self.reassign_class(self.settings["product"], self.settings["ifc_class"])
+    file: ifcopenshell.file
 
+    def execute(self):
+        ifc_class: str = self.settings["ifc_class"]
+        product: ifcopenshell.entity_instance = self.settings["product"]
+        predefined_type: Union[str, None] = self.settings["predefined_type"]
+
+        was_type_product_before = product.is_a("IfcTypeProduct")
+        schema = ifcopenshell.schema_by_name(self.file.schema)
+        is_type_product_after = schema.declaration_by_name(ifc_class)._is("IfcTypeProduct")
+
+        if was_type_product_before == is_type_product_after:
+            return self.simple_reassignment(product, ifc_class, predefined_type)
+
+        switch_type = "occurrence_to_type" if is_type_product_after else "type_to_occurrence"
+
+        return self.switch_between_class_types(
+            product,
+            switch_type,
+            ifc_class,
+            predefined_type,
+        )
+
+    def switch_between_class_types(
+        self,
+        element: ifcopenshell.entity_instance,
+        switch_type: Literal["occurrence_to_type", "type_to_occurrence"],
+        ifc_class: str,
+        predefined_type: Union[str, None],
+    ) -> ifcopenshell.entity_instance:
+
+        psets_to_reassign: list[ifcopenshell.entity_instance] = []
+
+        representations = list(ifcopenshell.util.representation.get_representations_iter(element))
+        for rep in representations:
+            if switch_type == "type_to_occurrence":
+                rep = ifcopenshell.util.representation.resolve_representation(rep)
+            ifcopenshell.api.geometry.unassign_representation(self.file, product=element, representation=rep)
+
+        if switch_type == "type_to_occurrence":
+            occurrences = ifcopenshell.util.element.get_types(element)
+            psets_to_reassign = element.HasPropertySets or []
+
+            element = self.reassign_class(element, ifc_class, predefined_type)
+            ifcopenshell.api.type.unassign_type(self.file, occurrences)
+
+            for pset in psets_to_reassign:
+                rel = self.file.create_entity(
+                    "IfcRelDefinesByProperties",
+                    GlobalId=ifcopenshell.guid.new(),
+                    RelatedObjects=[element],
+                    RelatingPropertyDefinition=pset,
+                )
+        else:  # occurrence_to_type
+            element_type = ifcopenshell.util.element.get_type(element)
+            # Handle element type.
+            if element_type:
+                ifcopenshell.api.type.unassign_type(self.file, [element])
+
+            # Handle containers and aggregates.
+            if ifcopenshell.util.element.get_container(element):
+                ifcopenshell.api.spatial.unassign_container(self.file, [element])
+            elif ifcopenshell.util.element.get_aggregate(element):
+                ifcopenshell.api.aggregate.unassign_object(self.file, [element])
+
+            for rel in element.IsDefinedBy:
+                pset: ifcopenshell.entity_instance = rel.RelatingPropertyDefinition
+                objs = list(rel.RelatedObjects)
+                objs.remove(element)
+                if objs:
+                    rel.RelatedObjects = objs
+                else:
+                    history = getattr(rel, "OwnerHistory", None)
+                    self.file.remove(rel)
+                    if history:
+                        ifcopenshell.util.element.remove_deep2(self.file, history)
+                psets_to_reassign.append(pset)
+
+            psets = ifcopenshell.util.element.get_psets(element)
+            for pset_name in psets:
+                pset = self.file.by_id(psets[pset_name]["id"])
+
+            element = self.reassign_class(element, ifc_class, predefined_type)
+
+            # Reassign psets.
+            if psets_to_reassign:
+                element.HasPropertySets = psets_to_reassign
+
+        # Reassign representations.
+        for rep in representations:
+            ifcopenshell.api.geometry.assign_representation(self.file, product=element, representation=rep)
+
+        # Keep IFC valid.
+        if switch_type == "type_to_occurrence" and representations:
+            ifcopenshell.api.geometry.edit_object_placement(self.file, product=element)
+
+        return element
+
+    def simple_reassignment(
+        self,
+        element: ifcopenshell.entity_instance,
+        ifc_class: str,
+        predefined_type: Union[str, None],
+    ) -> ifcopenshell.entity_instance:
+        element = self.reassign_class(element, ifc_class, predefined_type)
         if element.is_a("IfcTypeProduct"):
-            for occurrence in ifcopenshell.util.element.get_types(element) or []:
-                ifc_class = ifcopenshell.util.type.get_applicable_entities(self.settings["ifc_class"])[0]
-                self.reassign_class(occurrence, ifc_class)
+            for occurrence in ifcopenshell.util.element.get_types(element):
+                ifc_class_ = ifcopenshell.util.type.get_applicable_entities(ifc_class)[0]
+                self.reassign_class(occurrence, ifc_class_, predefined_type)
         else:
             element_type = ifcopenshell.util.element.get_type(element)
             if element_type:
-                ifc_class = ifcopenshell.util.type.get_applicable_types(self.settings["ifc_class"])
-                if ifc_class and len(ifc_class) == 1:
-                    element_type = self.reassign_class(element_type, ifc_class[0])
+                ifc_class_ = next(iter(ifcopenshell.util.type.get_applicable_types(ifc_class)))
+                element_type = self.reassign_class(element_type, ifc_class_, predefined_type)
                 ifc_class = element.is_a()
-                for occurrence in ifcopenshell.util.element.get_types(element_type) or []:
+                for occurrence in ifcopenshell.util.element.get_types(element_type):
                     if occurrence == element:
                         continue
-                    self.reassign_class(occurrence, ifc_class)
+                    self.reassign_class(occurrence, ifc_class, predefined_type)
         return element
 
-    def reassign_class(self, element, ifc_class):
+    def reassign_class(
+        self, element: ifcopenshell.entity_instance, ifc_class: str, predefined_type: Union[str, None]
+    ) -> ifcopenshell.entity_instance:
         element = ifcopenshell.util.schema.reassign_class(self.file, element, ifc_class)
-        if self.settings["predefined_type"] and hasattr(element, "PredefinedType"):
+        if predefined_type and hasattr(element, "PredefinedType"):
             try:
-                element.PredefinedType = self.settings["predefined_type"]
+                element.PredefinedType = predefined_type
             except:
                 # PredefinedType wasn't in the respective enum, assume it's actually USERDEFINED
                 # and set .ElementType / .ObjectType to the provided predefined type
                 element.PredefinedType = "USERDEFINED"
                 if element.is_a("IfcTypeProduct"):
-                    element.ElementType = self.settings["predefined_type"]
+                    element.ElementType = predefined_type
                 else:
-                    element.ObjectType = self.settings["predefined_type"]
+                    element.ObjectType = predefined_type
 
         return element
diff --git a/src/ifcopenshell-python/test/api/root/test_reassign_class.py b/src/ifcopenshell-python/test/api/root/test_reassign_class.py
index 196a60f154..f7b1024b6b 100644
--- a/src/ifcopenshell-python/test/api/root/test_reassign_class.py
+++ b/src/ifcopenshell-python/test/api/root/test_reassign_class.py
@@ -16,9 +16,14 @@
 # You should have received a copy of the GNU Lesser General Public License
 # along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
 
+import ifcopenshell.api.aggregate
+import ifcopenshell.api.geometry
+import ifcopenshell.api.spatial
+import ifcopenshell.util.representation
 import test.bootstrap
-import ifcopenshell.api.type
+import ifcopenshell.api.pset
 import ifcopenshell.api.root
+import ifcopenshell.api.type
 import ifcopenshell.util.element
 
 
@@ -95,6 +100,102 @@ class TestReassignClass(test.bootstrap.IFC4):
         assert len(self.file.by_type("IfcWall")) == 0
         assert len(self.file.by_type("IfcWallType")) == 0
 
+    # Switching between occurrence / type classes.
+    def test_unassign_type_from_occurrences_if_switching_from_type_class_to_occurrence_class(self):
+        element_type = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        element1 = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        ifcopenshell.api.type.assign_type(self.file, related_objects=[element1], relating_type=element_type)
+        element2 = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        ifcopenshell.api.type.assign_type(self.file, related_objects=[element2], relating_type=element_type)
+
+        element_type = ifcopenshell.api.root.reassign_class(self.file, product=element_type, ifc_class="IfcSlab")
+
+        # Type is unassigned.
+        assert ifcopenshell.util.element.get_type(element1) is None
+        assert ifcopenshell.util.element.get_type(element2) is None
+        assert len(self.file.by_type("IfcRelDefinesByType")) == 0
+
+        assert len(self.file.by_type("IfcWall")) == 2
+        assert len(self.file.by_type("IfcSlab")) == 1
+
+    def test_unassign_type_from_element_if_switching_from_occurrence_class_to_type_class(self):
+        element_type = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        ifcopenshell.api.type.assign_type(self.file, related_objects=[element], relating_type=element_type)
+
+        element = ifcopenshell.api.root.reassign_class(self.file, product=element, ifc_class="IfcSlabType")
+
+        # Type is unassigned.
+        assert ifcopenshell.util.element.get_types(element_type) == []
+        assert len(self.file.by_type("IfcRelDefinesByType")) == 0
+
+        assert len(self.file.by_type("IfcWallType")) == 1
+        assert len(self.file.by_type("IfcSlabType")) == 1
+
+    def test_unassigning_container_switching_from_occurrence_class_to_type_class(self):
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        container = self.file.create_entity("IfcBuilding")
+        ifcopenshell.api.spatial.assign_container(self.file, products=[element], relating_structure=container)
+        element = ifcopenshell.api.root.reassign_class(self.file, product=element, ifc_class="IfcSlabType")
+
+        assert len(self.file.by_type("IfcRelContainedInSpatialStructure")) == 0
+        assert len(self.file.by_type("IfcWall")) == 0
+        assert len(self.file.by_type("IfcSlabType")) == 1
+
+    def test_unassigning_aggregate_switching_from_occurrence_class_to_type_class(self):
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        aggregate = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcSlab")
+        ifcopenshell.api.aggregate.assign_object(self.file, products=[element], relating_object=aggregate)
+        element = ifcopenshell.api.root.reassign_class(self.file, product=element, ifc_class="IfcSlabType")
+
+        assert len(self.file.by_type("IfcRelAggregates")) == 0
+        assert len(self.file.by_type("IfcWall")) == 0
+        assert len(self.file.by_type("IfcSlabType")) == 1
+
+    def test_keeping_psets_switching_from_occurrence_class_to_type_class(self):
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        pset = ifcopenshell.api.pset.add_pset(self.file, element, name="TestPset")
+        element = ifcopenshell.api.root.reassign_class(self.file, product=element, ifc_class="IfcSlabType")
+
+        assert ifcopenshell.util.element.get_pset(element, name="TestPset")["id"] == pset.id()
+        assert len(self.file.by_type("IfcRelDefinesByProperties")) == 0
+        assert len(self.file.by_type("IfcWall")) == 0
+        assert len(self.file.by_type("IfcSlabType")) == 1
+
+    def test_keeping_psets_switching_from_type_class_to_occurrence_class(self):
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        pset = ifcopenshell.api.pset.add_pset(self.file, element, name="TestPset")
+        element = ifcopenshell.api.root.reassign_class(self.file, product=element, ifc_class="IfcSlab")
+
+        assert ifcopenshell.util.element.get_pset(element, name="TestPset")["id"] == pset.id()
+        assert len(self.file.by_type("IfcRelDefinesByProperties")) == 1
+        assert len(self.file.by_type("IfcWallType")) == 0
+        assert len(self.file.by_type("IfcSlab")) == 1
+
+    def test_keeping_representations_switching_from_occurrence_class_to_type_class(self):
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        context = self.file.create_entity("IfcGeometricRepresentationContext")
+        representation = self.file.create_entity("IfcShapeRepresentation", ContextOfItems=context)
+        ifcopenshell.api.geometry.assign_representation(self.file, element, representation)
+        element = ifcopenshell.api.root.reassign_class(self.file, product=element, ifc_class="IfcSlabType")
+
+        assert ifcopenshell.util.representation.get_representation(element, context=context) == representation
+        assert len(self.file.by_type("IfcWall")) == 0
+        assert len(self.file.by_type("IfcSlabType")) == 1
+
+    def test_keeping_representations_switching_from_type_class_to_occurrence_class(self):
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        context = self.file.create_entity("IfcGeometricRepresentationContext")
+        representation = self.file.create_entity("IfcShapeRepresentation", ContextOfItems=context)
+        representation.Items = [self.file.create_entity("IfcExtrudedAreaSolid")]
+        ifcopenshell.api.geometry.assign_representation(self.file, element, representation)
+        element = ifcopenshell.api.root.reassign_class(self.file, product=element, ifc_class="IfcSlab")
+
+        assert len(self.file.by_type("IfcRepresentationMap")) == 0
+        assert ifcopenshell.util.representation.get_representation(element, context=context) == representation
+        assert len(self.file.by_type("IfcWallType")) == 0
+        assert len(self.file.by_type("IfcSlab")) == 1
+
 
 class TestReassignClassIFC2X3(test.bootstrap.IFC2X3, TestReassignClass):
     pass