Some or all of the canon Undertale characters swap roles and/or personalities with or shift into the roles of 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.
If there is an in-universe reason for characters to change roles from their original ones, it doesn't fit in this type either. In that case, the AU is most likely in the Concept Change and Special Event types.
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