If this was high school ball, no way are we allowing the change.
I imagine the conversation would go like this:
ME: Sorry coach, you can't change your mind on substitutions once they are beckoned on.
COACH: waaaaa waaaaa waaaaa
ME: That is why you have assistant coaches. To keep track of things like the arrow. If thee was a problem, we should've been notified of it before YOU CHOSE to sub.
ASSCOACH: waaaaa waaaaA waaaaa waaaaaaa waaaaaaa waaaa waaaaAa
ME: WHACK!