I have a gorw light in my garage complete with reflector and Balast (balast is the control box thing - seperate from the reflector right?) I believe it is a MH and I will have to dbl check about the wattage. When it comes time to flower can you take out the MH bulb and replace it a HPS bulb of the same wattage? Not sure how these work but I don't want to fork out the cash for a whole new system. Any info is certainly appreciated.
Unless you have a switchable ballast then no you can't interchange bulbs. They do however make MH and HPS conversion bulbs that allow them be run in the other type of ballast.