Swap (チェンジ, Change), officially referred to as Swap to Mythra (ヒカリチェンジ, Hikari Change) and Swap to Pyra (ホムラチェンジ, Homura Change), is Pyra and Mythra's down special move in Super Smash Bros. Ultimate. It allows Pyra and Mythra to turn into each other, in the same fashion as Zelda and Sheik's Transform in Melee and Brawl.
Like Transform in Melee, Swap is a very fast switch between Pyra and Mythra. The move is fairly fast to complete, allowing Pyra/Mythra to act earlier than Pokémon Trainer's Pokémon after Pokémon Change, and having no cooldown for each transformation; unlike with Pokémon Change, however, Pyra/Mythra are granted intangibility for a shorter duration, starting from frame 6 instead of frame 1. Additionally, both characters maintain their drift when using the move, greatly circumventing potential momentum loss, especially in the case of Mythra swapping to Pyra.
Both have quotes they will say when swapping to the other:
After both Pyra and Mythra become playable in Xenoblade Chronicles 2, the player can freely swap between them during battle to use their unique abilities and special moves. This mechanic was recreated in the context of Smash. The names "Swap to Mythra" and "Swap to Pyra" are directly from the in-game menu prompt that allows the player to swap between them outside of battle.