Enter the demon

Paige makes a major blunder when carelessly mixing potions for Piper while secretly wishing she could be in Phoebe's shoes training for combat, and ends up actually switching bodies with Phoebe.  While the two sisters try to reverse the spell and struggle to harness each other's powers, they must battle a Zen Master's powerful disciple who has turned evil against his mentor and opened up a portal between two worlds.