basically what you need to do is go under object editor, pick create new unit, after that you select the type of unit you want to make. Select a hero unit for example the Blademaster. after selecting the blademaster as your new custom unit you simply scroll down till you find:
Art-Model File: Blademaster, Samuro
After you find this double click it and scroll till you find Peon or whatever unit you want to change it to. and boom! your new unit now looks like a peon (or whatever else it was you selected) but is a hero.