Welcome to Flyertalk 148ctp, welcome to the BA forum. Thank you in particular for putting BA's CEDR submission, it's the first one I've seen. Please keep us informed about how this works out, it will benefit other passengers.

I believe BA are incorrect. Yes, in the ideal world you would have told BA to put on your PNR that you accept the downgrade "under protest", but in the regulations there is very little wriggle room for BA in this area. You were placed in a lower class, and indeed you were reticketed, but that was inevitable in the circumstances fiven the two airlines and the PNR involved. And you've found the "comparable transport conditions" line too. So I think you have a strong case, and if CEDR fails to agree then I'm as sure as can be that a district judge will not entertain this. Otherwise every downgrade can be successfully achieved by the mere act of reticketing.

If, incidentally, you did in some way express your disapproval, even if verbally at the airport, be sure to highlight this. It is somewhat implied in the note they reproduced towards the end of the submission.

BA may have been on stronger grounds if they had given you several options and this was the one you selected. From the context above, that appears not to be the case.