(No images as the tumblr specifically requests nothing be taken from it.)
Blueberry is only 10 years old and takes the role of Coraline.
Swap!Papyrus is Sans's carer/gaurdian while still being his brother, essentially taking the roles of both parents.
Like in the film there is the "Other" family, in this case the other brother, who looks the same but his eyes are replaced with buttons.
Wybie is replaced by Asriel.
Wybie's grandmother is now Toriel and Asgore.
Muffet, Mettaton and Napstablook takes the roles of Mr. Bobinsky, Miss Spink and Forcible.
The cat it replaced with Flowey.
There is no Frisk.
Links not given in some cases as I don't know if they're using the UT, US or UF version.
Following the death of Gaster, Sans's and Papyrus's father, the two were forced to move into a new house as Papyrus could not afford the rent. Now being Sans's sole caretaker and having to work multiple jobs to make ends meet, Papyrus becomes overworked, stressed and has little time for his brother. Being only 10, Sans doesn't really understand what Papyrus is going through and misses the bond they shared before moving. That's when he finds the door.
Chara, classic Sans and fell Sans