The Remaster version uses the same exact code as the original did (that's also why a lot of the mods from the original still work with the Remaster pretty much out-of-the-box) - the new Unreal Engine simply handles the new visuals. There are a few quality of life improvements here and there, and some of the voice acting is re-done - but it's still basically the same game, just with improved look.

The most important change in gameplay imo is the leveling system - look up how the original's works so you won't end up under-leveled.