I've had this problem too, today. I rang EC to try and get 20E/F for our flight to MIA this sunday. I spoke to a v helpful lady who was unable to make the the seating change because the system has had a fault since sometime yesterday. She was expecting it to be sorted at about 1115 today (or that may simply have been because 1115 is T-48 for our flight; at that point the seating plan "is handed over to the airport", apparently).
She took my number and at 1230 left a voicemail confirming we now have 20E/F. MMB still shows our original seats and shows 20E/F as greyed out, so I phoned EC again. They were able to confirm that we really do have those seats and that MMB is going to take some time to update.
Hope that helps!