Just wanted to say thanks to everyone for their bits of advice and give an update on how it all worked-out . I decided not to use OLCI and just keep in pre-assigned seating.

My outbound flight (YYZ-LHR) was cancelled almost as soon as I arrived at the airport, so I got re-routed on *A by calling BA reservations call centre in Toronto (their line answered within 15 seconds). With the new routing, I was leaving Toronto 2 hours later, yet arriving in Berlin half an hour earlier than originally planned.

Coming back from TXL, the [Globeground?] check-in agent could not see the check-in note in the PNR, but after I insisted, she called someone and they advised her that it was in fact there. After that, she immediately gave me a new CE boarding pass for the TXL-LHR leg, but said that they were not authorized to process any upgrades whatsoever ex-LHR.

At the gate in LHR, I asked about the upgrade, but the lead agent told me that the next cabin was completely full on that flight. Being somewhat disappointed, I headed to my pre-assigned seat. About 10 minutes later, I was approached by a very pleasant flight attendant [CSD?], who asked if I could please come with her. She then escorted me to the new seat and said that she hoped that I would be more comfortable there ...

Well-done BA!