Blue Egg plus Wild Melon will shrink breasts, while Soft Mud reduces Femininity, to reverse the most obvious effects. If you also want to bump your hip and butt sizes down you can use Minoblood Wine+Juicy Peach.
The only truly permanent transformations in the game are ones where you bind your soul to one of the Seven (and Evergreen is supposed to offer one eventually as well but a divine TF can overwrite that) but currently there's only one of those available and the game will warn you that it's permanent before you go through with it. Everything else can be undone with enough alchemy and supplications to RNGesus.