My last DCA-JFK-LHR was changed to DCA-ORD-LHR. I had used a VIP so I was rebooked into A (I think) on the DCA-ORD seg and for some reason fully J on ORD-LHR. It didn't cross my mind to try and upgrade to F. I did get full miles but they eventually went back and adjusted them to my original Y booking class.