Should I replace the ballasts in my NOS MV yardblasters with some new HPS ones? I want to use the fixtures but I think MV is too precious to use for general lighting, and I happen to have some replacement HPS ballasts that aren't being used for anything.

If they are just cheap Regent fixtures I'd say go for it, aftermarket HPS ballasts will be better quality anyway.

Keeping a nice HID fixture going whatever you have installed in it is good.