Well, I know you can change the biome using WorldEdit. It's pretty easy to install, as well as Forge. I got it on a Linux laptop using YouTube tutorials in less than an hour, and it works just great!
I'm sure if you contact a judge, they should be able to tell you. As far as I'm concerned, it isn't against the rules. They didn't seem to state anything about changing the biome; after all, they do encourage you to be creative!