You can put a newer back on an older body - you just don't get any better interlocks than those provided with the backs that "match".
So if you have a Pro body, you can put ProS or ProSD backs on it. It won't offer double exposure prevention interlocks or exposure with dark slide still in prevention interlocks, but if you later get a ProS body, then the interlocks will function.
If you do get later backs, make sure the dark slide is of the same generation as the back.
Technically the backs come in two parts - an "insert" and a back shell. But unlike some other cameras, the back shell doesn't have much in the way of functioning mechanics - everything is in the inserts. Your back shells and inserts (and dark slides) need to be of the same generation for things to work.