You are describing Corruption of Champions there. You should try it.
CoC is ALL about changing something into something.
I'm talking about a way to increase/reduce attributes only, not a way to end up being a mass of tits and tentacles with 3 horse penises as legs, and eyes with nipples that shoot out flaming semen.
For example, the evil witch at the manor might reduce penis size and increase boobs if you choose the feminize option. Or the opposite with the mermaid. Small stuff like that, that keeps in the middle of being just a MtoF trans game.