Rarity takes Sweetie Belle to Canterlot with her on a business trip. Sweetie then proceeds to overthrow the royal sisters and take over all of Equestria.
Nobody really notices a difference.
Sweetie Belle obtains the Alicorn Amulet and tries to be evil for a day. She isn't very good at it.
The Great And Evil Sweetie Belle returns with a vengeance as the least threatening villain in Equestria.
Sweetie Belle takes a trip to Canterlot with her sister. Now she rules everything. The royal guards are thoroughly baffled.
Sweetie Belle sells her soul to a demon in exchange for very evil powers, and proceeds to (fail to) take over Ponyville.