Zack and Billy switching powers happened in the episode titled "The Mighty Morphin Power Rangers Movie," specifically the 1995 film. In this storyline, the Rangers encounter a new villain, Ivan Ooze, who presents challenges that require them to adapt and think creatively. To combat Ivan Ooze and his minions, Zack and Billy momentarily switch powers, allowing them to harness each other's abilities and provide a new approach to their fight.
This plot device is often used in superhero narratives to explore different facets of characters and to promote teamwork. It highlighted the importance of each Ranger's powers and skills, showcasing how they can work together and learn from one another. However, it's worth mentioning that in the television series, power swaps and transformations were not as common as in some other superhero franchises.