Storyshift

Storyshift is an AU of Undertale created by Reddit user Voltra. It's one of the earliest swap AUs. It has a comic too, but it has been discontinued in favor of a possible reboot.

In Storyshift, the roleswaps are
Papyrus > Toriel
Alphys > Napstablook
Chara > Sans
Asriel > Papyrus
Toriel > Undyne
Asgore > Alphys
Undyne > Mettaton
Sans > Asgore
Napstablook > Asriel
Mettaton (or in this case, his ghost form Mettacrit) > Chara