As this wasn't really a fantasy show, there wasn't much in the way of transformations. But here are some non-canon examples:

"Terrifying Tales of Recess": Cornchip Girl turns into a werewolf and back. There's two good scenes of this happening.

"Big Brother Chad": Vince has a nightmare about turning into a geek. Though I'm not sure if we have a category for this.

"Big Ol' Mikey": Mikey has a nightmare about turning into a giant. Though I'm not sure if they actually show him changing.

There is also an age regression clip.

Do you have the age regression clip?