Sure as long as the new bulb is for a mogul socket...and your using a 400 watt rated HID ballast. The biggest bulb I found in a medium base socket was a 150 hps, there could be more, I never looked for them really.

I have a plug between a 150 hps light bulb and the ballast, but I think you'll have to wire that mogul to the ballast tho.