The basic types of AUs.
Some or all of the canon Undertale characters swap roles and/or personalities with the other canon Undertale characters. e.g: Sans replaces Toriel. No new characters are added, and the story is relatively the same for the most part.
If only one character pairing is swapped, it doesn't fit in this type. In that case, the AU is most likely in the Character-centered type.
A mixture of a story, game, movie, TV show, or book, or personal characters and Undertale; A two unrelated universes/shows/etc combined into one AU.
Every single character in Undertale is changed while still remaining in the same basic story in some which way or form. AUs that only affect some of the characters are not found here. AUs that change only during the time Frisk is in the underground are not found in this category.
Some AU categories are part of this type:
As simple as it sounds, these are the AUs in which every character is the same person as every other character. Of cause this brings up questions like, is the Undyne in the Captain of the Guard role in Undynetale the same as canon Undyne or are they the Undynest Undyne in the multiverse (or an amalgamation of every other character, as she is "everyone"). We may never know.
These are comedic in nature and rarely make much sense beyond amusement value.
The species of all the characters have changed. Being changed into objects also works here.
AUs that are focused on a particular set of characters. These include headcanons, extensions, and flat out new ideas.
AUs in which 1 or more characters are replaced with real people.