if we are talking about FF7, then no, they have to re-do the engine from scratch, it won't be able to handle the current HiDef graphics used now, I don't see a problem in doing so either, as much as they could milk the cow that is Final Fantasy VII (which after 13 years is still being played by milions of people, me included) making a new engine, creating things as they were, giving the same story and quests AND finishing the game development in which they abandoned a few things and ideas cuz they really were short on time (delayed 2 times) I think that even if the game was released on 4 DVD9 disks, people would still flock it and pay even up to 100$ if not more, and some pervs would do that just to see Tifa's oversized fan-service boobs or Yuffie's flat chest :D
Square simply has no will to do it, they know it would bring them a lot of cash, they just don't want to...