Wasn't there a message on your change email confirmation saying that would be the case and only the email would reflect your new seat allocation .
The email says:
Hadders, just a quick one to say your change of reservation on your way from Newark North Gate to Doncaster has been sorted. Please carry this email with you as proof of your new reservation.
Your original seats have now been cancelled for this part of the journey, so if your tickets were already posted out or collected from the station before you changed your seats, ignore the old reservations printed on those tickets.
The tickets hadn't been collected when I made the seat reservation change so given the wording used it's not unreasonable to assume that amended reservation coupons would be issued.