Well... YEs and no. To actually change a hero's model is impossible. What you can do is to replace the unit and give him the items the old hero had, and the xp the old hero had. A simulated modelchange, if you will.
Copy the hero you want to have the modle changed, paste it change the model, and make a custom Chaos ability that changes the unit the the changed unit you just made. run at map intilization set current research lvl of chaos = 1 and give the unit the chaos based ability to change its model.