Underchange is an AU where all the characters have changed.
After another reset, Frisk sneaks into Sans' garage and uses his machine to go to a different timeline, but has been sent to an "Altered" world.
Flowey is made of metal and his pedals are spikes. He is much more evil than Undertale Flowey.
Toriel wears blue clothing instead of purple. She uses water magic instead of fire. Similar to Undertale Toriel, she will do almost anything to keep you in her home.
Sans is a humanoid with whitish-tan skin, a yellow jacket and pants, and yellow hair.
Sans' father Papyrus recently has been divorced by Toriel. Sans is scared of Papyrus. Papyrus looks similar to Underfell Papyrus. His scarf and the rest of his clothes are dark yellow and dark orangish-yellow. In this AU, Papyrus mean and abusive. He is commonly trying to impress Toriel.
Undyne is %20 human and %80 Tiger. She has 2 pistols. In this AU there is no Undyne the Undying.
Alphys is a tall African rich Lady.
Mettaton is a killer cherry with a saw. His second from, Mettaton EX is a killer apple with a sword. Mettaton NEO is a killer pineapple with an axe. He can shoot his prickles.
Asgore is a Blue Leprechaun who previously dated Alphys.