HPS--Using different bulbs

I have a 600W HPS light. The power unit has 600W written on it but I was wondering if in the future (for smaller grows) if I can use a 400W or less

Also can it take a MH bulb?

No you can not.You buy a 600 watt ballast and it can only use a 600 watt bulb and no switching an hps bulb for an mh buld unless you bought a switchable ballast and then you have to flip the switch before doing so or you need to buy a 600 watt conversion bulb then if you have ahps ballast you buy an mh conversion bulb you can use in that ballast but it has to be the same watts as the ballast ie 600 for 600 watt ballast 400 watt bulb for 400 watt ballast.Explosions of hot shredded glass and electrical fires are not good things