Honestly, if there was one monster who could take this kind of abrupt transition in stride, it would be Sans.

"Humans?" They've forgotten what it's like to live around other humans, in all honesty. Last they remember, they weren't very fond of them in principle, but maybe that just came from being around monsters so much. So often? So frequently. (They need to stop trying to get their tenses straight. It never works, and it just makes things more confusing.)

But still, Sans has a point. It is a kind of surface, which is better than being Underground, in some respects. In some respects.