They cannot change their decision. How would you enforce it if they changed their mind. The penalties are dead ball - live ball, they would not offset. So, presumably you would enforce the Roughing 1/2 the distance, then back them up 15 yards to the 16 1/2 for the UNS. Or would you do it in order of occurance - back them up 15 to the 18 for the UNS and then enforce the roughing 1/2 the distance to the 9?
I agree, once the ball was snapped, the decision to enforce the UNS on the kickoff was set in stone.