From the Super Mario Wiki
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Transform is the down special move of both Zelda and Sheik in Super Smash Bros. Melee and Super Smash Bros. Brawl. Using the move transforms Zelda into Sheik and vice versa. In Super Smash Bros. Melee the transformation is faster as both characters are loaded at all times; in Super Smash Bros. Brawl, however, the transformation is longer since the game needs to load the other character separately mid-battle. Taking damage while the move is in progress immediately cancels the transformation.
In Super Smash Bros. for Nintendo 3DS / Wii U, mid-battle transformations are removed from the game, and Zelda and Sheik are two separate characters. As such, both characters have been given a new down special move: Zelda's new down special move is Phantom Slash, while Sheik's is Bouncing Fish.