I believe he's saying master one aspect at a time, then build on that. Trying to do too many new things at once usually just leads to a person doing them all wrong. If you're comfortable with mag changes standing still, great, go ahead and start adding other elements.No proficiency moving? I've been moving my entire life.Master reloading before trying to reload on the move. I've been to quite a few classes and have seen people fall because they were trying to do two things, but had no proficiency in either.
Why learn something twice? If you are going to change mags on the move, practice changing mags on the move. After doing that, changing mags standing still will be easy.