Had a sort of similar thing happen to me yesterday DFW-BOS. Checked in had seat 8A (upgrades never seem to come through for this route), even checked online about an hour before heading to airport to see if any delays (my SAT-DFW is always delayed) and still showed 8A. Get to the airport and go to kiosk to print boarding pass and now in 18A. Called exec plat desk and she still showed me in 8A and showed I wasn't checked in, even though I was holding a boarding pass in my hand printed from their kiosk. I also noticed on my phone app that the check in button was active again and every time I clicked nothing took and it would just be active again. Turned out had equipment change and 20 people (all exec plat and plat) were all thrown further back in the plane for new configuration with MCE seats, and of course I guess my row was eliminated so got no extra room. But my boarding pass was still good so didn't get rejected, did lose the original seat that I had chosen though and too full to move me back up, did luckily have an empty center seat next to me so that was a bonus on this route.