What if you woke up one day and you were a Pokémon?

Pokémon Mystery Dungeon: Team Go-Getters Out of the Gate! (2006)

Directed by Kunihiko Yuyama

Charmander's voice calls out from the darkness if Squirtle is okay. The sleeping boy wakes up, and can't believe himself when he sees a talking Charmander. Charmander then points out that he's a Pokémon too. The boy takes a look at his reflection at a river besides him, and sees that it is true! He has turned into a Squirtle, and has no recollection at all how this had happened.