*Mostly the same.
*Just has more scars.

UnderCorrupt Jeff really is pretty much exactly the same in UnderCorrupt. He cares about Sans and Papyrus, and is pretty much Gaster's head assistant.

As for the scars, only the one on his lower right arm is actually from a Corrupt encounter. The rest of them... Well. There are a lot more dangerous experiments in labs when you're in the middle of a war, making weapons, rather than in a peacetime.