Of course, you would have to give Ranma/Ryouga/everyone else REASONS to 
want to change to their cursed form. Perhaps make it so that if they learn
the technique, they HAVE to switch to their other form say once every twenty
hours or something, sort of like how Odo from StarDrek DS9 has to switch back
to his true(liquid) form for a few hours every day. After all, it's no fun
if they don't have to change to their other forms occasionally, if they have
full choice in the matter.

	I was thinking that despite mastery of the art of shapechanging Hot 
water would still shift Ranma to his natural form and cold to his girl form, 
just with the training he would be able to change back by concentrating, and 
with a few years to get it habitual the water splash would simply cause a 
flicker as Ranma changed back the moment he changed.