It isn't easy: the more I think about it, the more questions I have.
For instance, as d.stu said, some races differ very little.
Another thing: while the halfkin<->beastkin change can happen automatically, I'm not sure about the race reassignment. Imagine if you have a seraph, you cast a Mutation spell on her and she loses her wings: should she become a human and lose her racial traits (and boosts)? Well, some players would find it cool, others would not.
Things would be different, though, if some boosts depended on your body, instead of your race. E.g. wings should give you more agility, no matter what you are.