LadyTShay

Best answer as chosen by Asker / Ratings
Sims that existed before the expansion pack have a potion that will let them change that. You can have one of those sims (if controllable) put the potion down and get the sim you want to change to put it in their inventory and then have them drink it. If the sim isn't controllable you can use the boolprop testing cheat and make the sim controllable. I've done it that way before. I don't know of any other way unless you download a hacked buyable potion from somewhere.