Just to echo BotB's comments. I was actually booked on the FRA-LHR service some time back, when it was a mere 767 with bulkhead seat 2A (and 2B blocked off). When it went to an A380, this showed as being my seat right up to departure, but instead of my name being in a yellow box at the top of the seating map, it was in a white box which could not be changed. If I hovered the cursor over it, it was shown as "unable to change".

I rang GGL and asked them about it, they said the service had been under airport control and that unknown to me they had allocated me 10A. They asked me which seat I would prefer, I asked for 15A. A long hold later and they came back and said it was all confirmed (they didn't mention 15B but it was left empty), but any further changes had to be done via them. At OLCI I could not change seat, but 15A was waiting for me, so I printed it. I'd slightly disagree with BotB, if PYOBP gives you a seat you like, I'd print it in order to make it slightly less likely you would lose the seat to someone else. In the event there were plenty of seats available on the service to suit all preferences.

I felt somewhat silly about this since I'd never normally ring up BA just to change a shorthaul seat, but anyway.