Forking is totally legal. You need to do some things however. Don't call it OpenJDK since OpenJDK is copyrighted. Don't remove the Copyright inside the files.

Forking is totally legal. You need to do some things however. Don't call it OpenJDK since OpenJDK is copyrighted. Don't remove the Copyright inside the files. License new code as the same license as applied to OpenJDK. If that is happening you are completely free.