If it's an old school magnetic ballast...do not go mixing the wrong bulb to the wrong ballast.

There are some magnetic ballasts that have dual function capabilities...I'm thinking though if you had one of those you'd already know it.

Going the way of the digital ballasts will givea grower more flexibility as one of the key features of that type is being in being able to safely run either MH or HPS. They're a damn site cooler than magnetic ballasts too.