I worked a lot of rec ball and to years of middle school and JV ball prior to moving to varsity. Even so, I learned a lot during my first month of varsity ball. Certain situations will arise that you haven't seen before. But they're the type of things that you can't learn any other way, except to be on the floor during a varsity game.
Last night, I worked a scrimmage between two of the top 5 teams in NC. Over 800 fans showed up! For a scrimmage! It was like a playoff game! There were also 12-15 officials there, 5 varsity and the rest, JV officials. I worked the 1st qtr with my regular crew and nailed several hand checks.
It was quickly apparent that some of the JV officials were ready to jump onto the floor, while others wanted no part of it. So, when you're ready, you'll know.