Hi,
I have had this a lot recently but all was resolved for me on Saturday by a helpful agent. The problem occurs when your seat in the previous booking has not been removed. This therefore blocks any further seat selection using Manage My Booking online though the agents can do it on the phone. Because of the nature of upgrades and flight changes this is a common error.
The solution is to have an agent remove the seat assignment from the original flight and you will have full access to your new flight/cabin seat map in MMB again.
As an external company manage the seat reservations, the systems are not linked in a way that allows both to be updated at the same time.
Hope this works for you
