An object from a given class (which doesn't know about alter egos) will continue to access the object body (if that's what it does) with no interference. If an Alter-based class initializes and uses that object, a corona of alter egos will be attached for the Alter-based class(es) to use. The original class will know nothing about that and continue doing what it did all along.
Where's the problem?
Update: I'm not trying to make old code use the alter technique, that's the next stage :)