I'm a ref and yes you can change a judgment call. I had a coach ask me a great question one time when I called a takedown real quick..he said "did he have control beyond reaction time?" I thought for a sec and said no he actually didn't. Changed my call. Coaches appreciate honesty from referees and just because it's the call they made, doesn't mean it was right. Referees are hiding behind the term "judgment" too much to justify themselves. If we want to get better we need to get it right. Sometimes we make a call in a split second and realize the mistake. Nothing wrong with fixing it.