I don't understand why you need to change the date on the ticket coupon. The coupon is valid on any OW flight on the route regardless of what date is shown on it. (Provided that it is before the "Not Valid After" date). Just turn up at check-in with the coupon and it will be accepted. Change the reservation date simply by calling BAs reservation office.

If you are having to get a reissue done for reroute then get all the remaining segments issued open-dated (as IMHO you should have done in the first place) and make the reservations separately.