Handling Member Objects in loadobj() that no longer exist

I've got a situation where ObjA had a property of type ObjB. ObjA was then saved to a file. Then, ObjB was moved to +PackageA.ObjB. Before the loadobj() method of ObjA fires, it tries to instantiate ObjB but can't find the class definition for it because it was moved and is now PackageA.ObjB. How would I handle backward compatibility for objects that no longer exist? Do I just live with the warning:

Warning: Error loading an object of class 'ObjB': Its class cannot be found.

Do I need to still maintain a definition for ObjB? - probably I'm guessing eh?

Thanks,