I've always toyed with the idea of a multi-personality character, that actually could switch and do things better in one mind than the other. (Completely unrealistic, but so are dragons.) This offers a good means, but lord those Will saves are high. Any particular reason for the High Saves? I mean from a player's point of view that makes it all the better...

Oh also, when changing are hit points received? Or is the creature just unstable while under 1/4 of the Hit Dice, bouncing between one form and the other?