The graphics and the audio are both updated. The original didn't have the smoothing and fading in the graphics; it was almost just solid blocky colors. It's still lo-fi but not nearly as lo-fi as the original.
They did a really great job updating the graphics in a way that (in my personal opinion) makes them look more appealing than the original, but still makes them look like a classic videogame.
Yes, a trap a lot of HD remakes and fan-made HD mods fall into is that they upgrade sprite or texture resolution but keep animation framerate or geometry the same, which creates a janky/disjointed look that I can't stand.
