Fixed 22248 -- Fixed backwords model renaming

The context manager is an obvious oversight, but I'm not sure that I need to check for state changes after a backwards migration.

As far I can tell, the state dictionary is always migrated forwards regardless of the "direction" the migration is running towards, so state changes are properly tested in lines 417-422 of the same test.

Still, I may be missing something fundamental, so I'm waiting for your feedback.