Changing test dates?

<p>Has anyone had any experience signing up for a future ACT test and than showing up at an earlier date? I forgot to register D until it was too late and the ACT people advised that we register for the December test but show up early this Saturday to see if there is availability. I was told that it is generally not a problem if you are at the test center early and have proof of registration for a future test. Has anyone done this?</p>