Transform is a move found in Super Smash Bros. Melee on the GameCube. Princess Zelda can use it turn into Sheik and vice versa by pressing down-B. This isn't considered an attack, as it will not harm any of your opponents. It will also take a little time until it's completed, and it can be discontinued if you're attacked.
Zelda performed the move in Super Smash Bros. Brawl as well.
In Super Smash Bros. for Nintendo 3DS/Wii U, the move has been retired as transforming characters have been separated.