On two different flights from DFW to San Jose, Costa Rica, we had confirmed seats in economy at the bulkhead exit row. On both times we were moved back to other economy seats. The second time I even checked on line to see if we still had those seats before we went to the airport, which we did. No one could give me an explanation other than it might have been a "change in equipment", which it wasn't. We're platinum flyers, but we were flying on frequent flyer tickets. Now I'm afraid to reserve those seats because I'm afraid they are selling them to people willing to pay for preferred seating. Has anybody else had this experience?