It's not guaranteed to be easy to port. Each console has its wrinkles. The PS4's RAM has scads of bandwidth, but relatively high latency. The X1 has RAM much slower than GPUs normally have but that's obviates by an ultra-fast but manually-tuned lump of cache. Getting a port to run smoothly isn't something the intern does in an afternoon. Genuine effort is required.

What's worse, though, is the QA. Once you've done the port you need to make sure that it runs acceptably fast at all points through the game and at every point on the map in every combination of vehicles and if you're being chased by the cops or not chased or in a mission or not in a mission and so on. For every combination of game state, which is enormous in an open world game, some human often has to play the game into that state and confirm that nothing weird happens like slowdown or you vanished inside a solid object or the sky turned purple or any other obscure bug.

It's all tractable, and I think it's more likely than not to happen, but it's not as trivial as you suggested either. Far from it.