[Newbies] Remove old instances of a Class

How can I remove old instances of class that have been created while playing around in a Workspace?

I am developing some code that needs to go through and deal with allInstances of itself, and I just seem to have old versions hanging around that don't seem to be getting garbageCollected. Furthermore, I am expecting certain types of objects in some of their instance variables, and as I have changed my code, these objects have changed--but not in these "obsolete" objects, so I have had to manually go into those instances and "fix them up" so that my allInstancesDo loop would work!

I have tried closing the Workspace, followed by Smalltalk garbageCollectMost, but to no effect.

Thanks,

Rob