Okay, I have a solution for you. First you make a skill, that causes the transformation. And let it activate common event A. Next make common event A changing his class, graphic, adding skills (not removing skills, that makes it a lot harder to revert it). Then make common event B and make it a paralell process. Be sure to activate a switch that triggers common event B. Let common event B change back the graphic, remove the special skills etc.
But this will work for the original hero. He will get the experience and he will level up, there will be no "hero@", as you called him. Only his "look" and "powers" change by doing it this way. I tested it, it works. Is it that what you wanted?
EDIT: If you are changing class, be sure, the other class can equip the same equipment, otherwise he will unequip it as soon as the fight is over.