Kind of experimental. The idea is that it will reassign class e.g. from IfcWindow to IfcWindowType (or vice versa) and will keep the IFC representations and property sets.
Example - https://imgur.com/a/X7MHR0s
When PredefinedType is not found in the related enum reassign_class is considering it USERDEFINED and trying to set .ObjectType. The problem was it was doing for type classes also, when it should have set .ElementType instead.
Why it crashed BlenderBIM - in IfcClassData.data["has_entity"] we store currently active IFC entity which is recreated during `ifcopenshell.util.schema.reassign_class` making old entity invalid. Since `root.reassign_class` was failing in the process, related BBIM operator was failing too, IfcClassData wasn't updated and removed entity was accessed from UI leading to crash.
Andrej730