I'm bothered by this because the question and answers assume that cheerleaders can't be fat in the first place. A Google search says this depends on where you look. Here's one example: https://www.pressreader.com/.

That said, I feel like this has been done before though I can't point a finger at a specific case. Many works of fiction have a quick transformation that teaches a character a lesson about who they were before that corries forth as the physical realities return to normal.

If having the character become fat has a real purpose to your story themes and plot, do your research and find betas. If it was just an easy transformation to work with, I agree with your decision to change it.