Underswap was more defined, characters that didn't have obvious swaps or where there were two potential swaps (like MK, W. D. Gaster, Riverperson, Asriel, Chara and Frisk) were swapped with other characters. The now established canon has Monster kid swapped with Asriel and W.D. Gaster remains unswapped. There is however some art relating to an Underswap where MK and WD are swapped, think of it as an AU of an AU. (These AUs may work on Chara and Asriel having swapped roles instead of the canon Frisk and Chara swap as Frisk is seen unaltered in some of the art.)
Here is an MK art.
And here is a WingKid example.