Well it seems that North figured out the meaning of 4♠ before he finished executing bidding his bid, so IMO I would let him change the bid, but according to the regulations it seems ok to decide on the contrary. I would say that there is UI but there seems to be a remote chance that anyone can benefit from using them. Its normal that people forget conventions and how many times have you seen people handling 5♣ and then changing it to 5♦ when anwering 4NT (when changing from 3014 to 4130). Would you only allow the change if players didnt see the 5♣ bid? I think you should change the regulations B) :) .