Sans look like the exact same, expect his hood on his jacket is up. Instead of just one of his eyes turning blue when he uses his powers, both of them turn red.
Before the events of Undertale, Sans was with Papyrus, living the exact same as usual. But when the sixth human came, he thought that all monsters were evil, and also thought he was ordered to kill all monsters. The sixth human only killed some monsters in the Ruins, killing an entire kind of monster, and Papyrus at the beginning of Snowdin Forest. Out of anger, Sans killed the sixth human. Some time later, Sans was felling lonely, so he tried to bring back Gaster, but doing so, a demon from another AU to fuse with his soul.
Sans still likes to make puns with Gaster, though his puns aren't much great as they used to be. Instead of going to Grillby's, he makes his own food, which is usually Pizza. He doesn't go to visit Toriel, so he never made the promise, but he still sees if the human is bad or good.