Cardcaptor Sakura Season 1 Episode 32

Sakura, Kero, Shaoran and ...

During an attempted card capture, Kero and Syaoran were holding down a card (CHANGE) that took form of an animal. Sakura turned it back into a card, but because they were both touching the card at the same time, Kero and Syaoran swapped bodies. Kero tried to make it though a day as Syaoran without anyone noticing, but didn't do very well. At the end they finally change back when Sakura used the CHANGE card.