Why not allow class retraining? What if I started out a street urchin rogue but saw the light and want to become a paladin (but without multiclassing) leaving behind my roguish ways? It is even weirder since you can retrain your multiclass (seeing as those are feats) but not the other way around.
That would technically be a rebuild rather than retraining -- but it would be a good idea for the final book to give some guidelines about that. I am sure that many GMs will house rule this in anyway.