HID ballasts

So.. I was perusing my local craigslist and someone has a 1000w MH lamp+ballast blah blah all ready to go. Sent him an email got a pic and all that jazz. The problem is that I want a HPS bulb, not MH. I've been browsing the internet trying to solve this question and I can't:

Can you always switch out MH and HPS bulbs (assuming they're both 1000w) regardless of the ballast?

I'll probably buy it anyway as it's only $50.

There is some conversion bulbs out there on the web. So you could buy a 1000w hps conversion bulb that will fit on you're MH ballast. Just make a little search on the web you should easily find some.