I know this is a commercial product, but I wrote a game in which the entire cast is transformed into genies over the Christmas break. I'd link a youtube playthrough but none of the Let's Players are up to that point yet.
https://store.steampowered.com/app/1...luna_and_Brie/
The scene itself is about 10,000 words long, it takes place over a few in-game days. Here's some images of two of the characters in their genie forms:
https://imgur.com/a/gHtWVgV
There's also a similar scene themed around demonic transformations, and they turn into mermaids quite a few times. But the genie event is really significant in the game. It happens right at the end of Chapter 3.