As BA.com refused to let me do anything, I called the Gold line to ask for help on a return HBO to DUB in July.

The agent told me the seatmap had been locked out and she'd have to see if she could get it released. The explanation when she returned after a couple of minutes, which ties in with some of what's being reported here, was that status members' bookings were being individually updated to allow for free seat selection - which I imagine means (a) this is going to take a long while and (b) my update was done on the spot.

When I now go back in MMB, I have the seats allocated by the Gold line, but I get free seat selection if I want to change them.

Based on this, and in contrast to c-w-s's advice, my experience therefore suggests calling may work better in many circumstances.