In the Smash Bros. series there have been two characters who can change from one fighter to another. However, as part of a new standard for the franchise, Zelda and Samus will no longer be able to change into Sheik or Zero Suit Samus.
Typically, Princess Zelda could perform a special move that would change her into the mysterious Sheik from Orcarina of Time. For now on, this ability is replaced by summoning a phantom knight who rushes forward to attack. Other than causing damage, the knight is also able to block incoming attacks. Fans of Shiek shouldn't fret, though. The powerful ninja will be in the final roster for Super Smash Bros.
Samus's armor will no longer fall off when using her Final Smash, but, just like Sheik, Zero Suit Samaus will be playable as a separate character. Zero Suit Samus also comes with some tweaked moves thanks to the decision to give her jet boots. This increases the power of her moves as well as her jumping distance to make her less of a weak character while still keeping her limberness.
The decision to fully separate Zelda and Samus's dual identities will certainly create mixed reaction among veteran fans, but hopefully there will be enough diversity between each character's parallel form to make them welcomed "additions" to the Smash Bros. franchise.
Source: Nintendo Direct