I didn't notice any re-drawn art. Are you sure? The only thing I saw was that it seemed converted into a higher resolution.
You're probably right. I assumed that since it's 2D, you would need to redraw the sprites to avoid to looking like garbage.
But looking back at the footage, it looks like it the scale of the world has been increased.
I do notice a big difference though. It should run a lot better on modern PCs now.