KittyKat, I think you'll find that many doors will come "pre-hung", meaning they're already mounted to a door frame with hinges in place. It's obviously the way to go in a new-build situation. But I'd suspect that some contractors would rather do the same on a retro-fit - just pull out the entire old door set-up (perhaps saving the trim for reuse) and pop in the new.
Not saying at all that what you're wanting to have done isn't viable - you'll just need to find someone willing to go that route.
Bill
