I don't see this at all. The base rule is that the use carries over directly (which may be what you are referring to if he would change to a form with more Body, etc.) The optional rule is to figure the percentage, which I think negates your argument. If you're at 50% of body in form 1, then form 2 will be equally hurt at 50% of that form's body. And a third option would just be to have all the forms buy the same body, stun, and end. Then there is no "healing."
I'm not saying don't use the MP route, just that I believe the rules already cover your particular concern.