Try these. Only use one at a time.
+PropertyRedirects=(OldName="/Script/MyGameName/BP_DoorActor_C.Length",NewName="DoorActor.Length")
If that doesn’t work, you can be more forceful.
For this, you need the exact FULL path of your BP.
+ActiveGameNameRedirects=(OldGameName="/Script/MyGameContent/Blueprints/Doors/BP_DoorActor_C.Length",NewGameName="DoorActor.Length")
Another thing you could try is reparent to the base class of the C++ class and then reparent back. You’ll lose all the defaults values of anything in the C++ class though (edit: … in any instances).
I have all my redirects in the [/Script/Engine.Engine] section and they all work fine.
edit: Actually, you don’t need the full path in ActiveGameNameRedirects. If you have nothing else called DoorActor, you can actually just use “BP_DoorActor_C.Length”. Try that. You could also try “BP_DoorActor.Length”. The _C is usually for generated classes. So you may not need it.
+PropertyRedirects=(OldName="BP_DoorActor.Length",NewName="DoorActor.Length")
or
+ActiveGameNameRedirects=(OldGameName="BP_DoorActor.Length",NewGameName="DoorActor.Length")