Apologies for the presumably duplicate thread. Unfortunately I feel like there are many stories like this already out here. I just want to add mine to the list. Here is the note I submitted on Delta.com:

I had a very poor experience with the upgrade process today. I was flying on a rather expensive, last minute (4 days ahead of time) ticket from Indy to NYC. Up to 45 minutes prior to departure the upgrade list showed me as #1 for 2 seats. After getting through security in Indy and heading to the SkyClub I noticed that I had been removed from the list and 0 seats remained, presumably having received the upgrade. I asked about a new boarding pass and was then informed that "someone had purchased 2 last minute first class seat. Okay, I understand that Delta will sell tickets for as much money as possible if someone shows up. However, it seems odd to me that this happened at < 45 minutes to departure. Also, this seems to have happened AFTER my upgrade on this flight was processed. How is it possible that the seats are upgraded but still made available for sale? Here is where the real problem occurs - I've now been upgraded from my purchased and selected seat, just to have that upgrade rescinded. However, now my original seat has been reassigned. So now, instead of 4B (which on the Embraer 175 is, in my opinion, the best coach seat available with unlimited leg room), I'm smashed against the window a few rows back. At this point there are no aisle seats available anywhere on the plane. I asked the agent (who was apologetic) why I couldn't at least have my original seat and she had no response. I'm not sure how someone else could be put into my seat and I could be reassigned, yet she couldn't go back and allow me to sit in the seat that I booked at purchase and move that person to my "new" seat. There was no aircraft configuration change in this case that would have moved multiple people around - this was simply a case where I was upgraded during a normal timeframe, delta decided to sell the remaining first class seats despite the late hour and rescind the upgrade, and during the window between those two events give my original seat away to someone else. In my mind, this situation is simply unacceptable. This is completely against the common sense that the delta commercials promise not to let the rules get in the way of. If you upgrade someone, don't sell that seat out from under them. If you somehow manage to do it anyway, don't take their original seat away from them, or empower the agents to make it right.