They would be wasting their time. EC261 only applies to the operating airline (which does not seem to have done anything wrong here) and on flights from the EU or to the EU on an EU carrier neither of which is the case here.

As NickB points out, this is not a case of downgrade but a case of - effectively - falsely representing the service sold. I would have thought that the OP would have had a fairly strong case following such a complaint but whether this could ever be worth it considering the very minimal money that would probably be repaid is questionable. I almost wonder if it might make more sense to just complain to a consumers' association or something like the "ask Alex" section of Business traveller hoping that fear of bad publicity gets some reaction.