Most likly the ballast would be fine and the bulb would burn out. The HPS needs an ignitor to run but as I understand it is basically the same besides that. If you have a MH ballast it is good for only the MH. If you got a HPS ballast you can bipass the ignitor and run a MH. Some people say they can run a MH bulb on a HPS ballast with out bipassing the ignitor but I do not know the long term effects.