Im a new user, thanks for the reply. Suppose we put aside the codeshare aspect of friend's problem. So situation is passenger bought SEA-LHR in paid first on BA, BA canceled the flight with six weeks notice, available nonstop alternatives are BA flight in first 6 hours earlier, or Delta flight in business at the scheduled time. Is this then a standard EC261 downgrade? The only convenient/similarly-timed flight is one with a downgrade, so passenger gets 75% downgrade compensation per EC261, no matter how far in advance the airline cancels the flight -- because the downgrade benefit does not depend on amount of notice. RIght?
I share your intrigue about how downgrade rules apply to situations where the entity processing the downgrade is someone other than the operating air carrier. Maybe my friend should begin, then, by contacting BA. BA YouFirst is a better team to deal with than AA Reservations. BA may say that they won't do anything in response to their cancellation, and my friend can only deal with AA -- but if BA handles the cancellation and reaccomodation, then my friend would seem to be within the plain language of the EC261 downgrade rules per your quote.
My friend already flew the outbound segment, btw. All the discussion in this thread is about the return (back half of the rt). I think it's more common to talk to operating carrier when journey is underway, versus before journey begins, right?