Courtesy Update
Our official BA Lurker confirmed that I can MFU mid way through a trip if there is availability but that my flight showed none currently.
At T-24 (in a restaurant with my GF and her mom (Cactus Club - one of the best chains I have ever eaten in FWIW)) I checked her in and tried to move her to 11K next to me. It refused as I had prepaid for 12J and therefore was only eligible to move seats at the airport - go figure...
So anyways I poked around a bit more and frankly I can't remember what I did - I
suspect I accepted this then went into the booking and modified her seat assignment but I couldn't be sure. I ws able to move her to 11K next to me. The check in said she was in 11K, the MMB said she was in 12J. So to be extra safe I printed her boarding pass with her in 11K.
I then went to my own MMB and frustratingly it offered me the same £399 CW upgrade it had withdrawn. It goes without saying that I am far too chivalrous to accept this without being offered it for her so I quickly confirmed my 11J and printed my BP.
At the airport tomorrow I'll query this iniquity
.
Thanks to all involved for their help - I am slowly feeling my way to becoming a more experienced BA flyer. Next flight back to VS next Thursday for another EWR run...