All cutscenes were remade, all Graphics assets were improved and enhanced using UE4. I have original 360 and Ultimate both in library. I played both back to back and difference was night and day. Gameplay still feels same in campaign with same A.I but multiplayer feels very different and quite modernised. Ultimate Edition multiplayer is better than Gears 2 and 3 in terms of animation and gunplay.
Its still keeps the essence of original intact but lot of stuff is remade using UE4. Original game used UE3 not UE4.
Reloaded is upgrade over Ultimate using UE5.
Engine upgrade alone is huge. UE5 brings lot of new graphics features that are not in Ultimate Edition.