That is shameful! And not as uncommon as you would think. IMHO, it ranks right up there with the upgrading of non-revs, leaving PMs and GM's in the back, regardless of fare class (Simply Not Good Business). LAX is notorious for this . I won't fly out of LAX WITHOUT my confirmed upgrade.

IF YOU FEEL THAT YOU HAVE BEEN TRULY WRONGED, and you want to stick to your guns, and don't care what they think of you after you leave (not an airport you regularly fly in/out): If they are unable to satisfy your polite request, then politely ask if there are any non-revs on the flight. If yes, then politely ask again if there isn't something that they can do to get you out of that dreaded middle seat, maybe even back into your original assigned seat. If still no, then politely ask the gate agent for their name, and the names of the other people working the flight, and ask for a supervisor. Away from the gate agent, politely explain the situation, and ask for help. In some instances, the supervisor can "make it better". If not, smile, thank the supervisor, mention that you will follow-up on this when you get home, thank the gate agent(s), get on the plane. If possible, look at the manifest. If not, and you still want to pursue it (and you have the time), talk to a supervisor at your destination airport. At this point, unless you really feel that you have been wronged, give up! If it it's an equipment change-all bets are off.

Sorry for the rant, but the sad reality of having to reluctantly leave Delta after 20 years is frustrating, sad, and P****** me off. Apologies in advance to any who may be offended.