Flew AF last week, having selected an exit row on the A380. At check in (DL on domestic segs) I was issued different - much worse - seats in the same row. I complained about this at the DL check in, but they told me they couldn't re-assign me to my original seat b/c it was now occupied, and to talk with AF at the gateway airport. On DL I received one of two eligible UG on the connecting sectors, then I arrived at the gateway. I did approach the AF agents and requested my seat back. AF agents were apologetic, but couldn't find me my original seat or an equivalent exit. They told me another pax had paid for the seat assignment. I showed them my printout, from some time previously, showing my exit assignment. They did not deny I had this assigned all the way from months prior up to the day of departure. They told me the system had changed and that not only had I been moved but that other pax had been assigned to seats (90B) that don't exist on the AF380. All well and good, but that didn't account for my unexpected displacement. Also, then they told me the usual, that seat assignments are not "guaranteed," they're a request etc. I boarded and accepted my fate. You can imagine the low comfort levels of this new seating arrangement. At least the flight was faster than usual so the torment was limited. I wrote it in to DL when I arrived and was awarded 10k SM in comp. I'd rather have had my original seat. This means, DL Medallion status is actually worse than no status at all, because a no-status pax is not likely to be shifted and in any case, most likely not to a much worse seat at the last minute as I was. This lack of value is just more prep for the "enhanced" SM program, to encourage the customers to adopt the right frame of mind.... where they know it's worse to be a DL Medallion than not. Take home: check your AF seat assignments on checkin if you have exits/EC on later sectors. Probably better to address it sooner and not at the gateway if you can.