Yep, the illegal position change by A80 is what I was looking for. That's a dead-ball foul, so we'd sound our whistles and interrupt the play clock. This foul is listed in the rules under False Start (and I'd write it up as such). But, personally, I'd announce it as, "Illegal position change, number 80, offense; number 80 established himself as the snapper, and then moved to another position. By rule, once the snapper is established, he may not move to another position. That's a 5-yard penalty, and the Try will continue."
I've had that exactly once in my career, on a swinging gate like that. Nailed it.