Lumatek Digital ballast Question

Ive got a 120 volt 1000 watt ballast, and they say u can use MH in them as well as HPS. i only got it for the hps but was wondering how this is done?
with a diffrent lamp, or just switch the bulbs?

buy a metal halide bulb and replace the hps bulb with it.

ya i jus found that out right after i posted this....i feel fukkin stupid