Would it be wrong to submit one of my published fanfictions to a writing contest?

Recently, my university announced that they were having a writing contest with cash prizes for the top three submissions. I was thinking of taking one of my fanfictions, changing the characters to some OCs so I don’t have to deal with copywrite complaints, and submitting it. It’s a cute little one-shot, but I’m afraid that people will view me as immature since it as originally a fanfiction…

I think it’s just me being insecure, since I’ve never considered entering a contest before… But I would like to validate my writing, even if I don’t win.

You should check with the university. In somewhat similar situation, that can count as academic dishonesty. For example, you cannot submit an essay you wrote for one class to another class for credit. This is something you should check for sure.