I wonder what Mr Banner Q.C. might think of BA opening CE middle seats when it is convenient for them to do so.

Seriously though, I think this is a cheap move. If BA compensated CE passengers for the downgrade, or made some gesture and offered free rebooking or cancellation then perhaps arguable. These days of Covid and Social Distancing, I reserve CE for the extra space on board ,since all the other extras I have with my shiny card anyway. I would be none too amused to find the middle seat allocated and would indeed consider this a downgrade and pursue it as one, since the free middle seat and the $10 "meal" is the only difference to Economy.