I know from experience. No seat is guaranteed. I used miles to purchase first class on AA. Someone else with money wanted it. They were strong arming me to move me back to exit row and give this big shot my seat next to my wife. I had to exit the plane and flight was delayed 45 min while we sorted it out. I won but others missed connections because of this. I had 4 hrs layover and didn't care. So don't tell me it can't happen. Especially with a miles ticket.