Equipment changes are fairly par for the course.

CO sets the baseline for domestic service as Domestic F. Some aircraft happen to be a little nicer, but that's an 'over and above' -- not a different baseline.

Now, if CO sold a "premier" service -- using the BF seats -- and then swapped in a domestic 757, you'd have a point. No different than if they swapped in a domestic 757 for an international 757 flying EWR-SNN, for example (because in that case you would have paid for BF service, had a BF ticket, and not gotten the service).

Steve