PMFJI, but I have just had to do this very thing.

If you haven't already done the hacking and haven't already made new subclasses from the new class library, move the class back where it was so all the subclasses (and/or forms) will work.

Using the Class Browser, open up the original (source) class library.
In this same CB instance, open up (View additional file) of all the forms and class libraries with classes that inherit from this class.
Open up the target class library in another instance of the CB.
Drag the class you want to move from the source CB to the target classlib in the other CB.
All the subclasses and forms will now inherit from the new guy. Pretty cool, huh?

I tried doing this with only one CB instance, but it didn't seem to do anything.
HTH