Bicostal

Sarcasm aside, the interpretation used by some that the policy is that all unclaimed seats are released at T-15 is incorrect. The policy is stated that IF you are not in the boarding area t T-15 your seat MAY be released. This also applies to not being on board at T-10, I believe. Obviously if people are still boarding, no GA is going to release seats of passenger who are checked in UNlESS they a confident that they will misconnect. The ones in line to board aren't going to misconnect.

Once boarding has been completed inside of T-15, the GA can and would likely release remaining FC seats for upgrades and Y seats for stand by passengers.

In the case of this OP, the GA released seats and had the flt not had an IROP would have gone out on time without the misconnecting passengers. In the OPs case it appears that when IROP happened, the GA reassigned the original FC passenger to his seat and reassigned the OP to his original Y seat since now the misconnecting passengers was no longer at risk for the misconnect.

GAs are pragmatic and focus on getting flts out on time. It's not a perfect science but nonetheless, once a seat is reassigned I find it problematic to unassign it without consideration of the passenger and policy. In this case, as well as in cases others have observed, Delta seems to undue upgrades in tsome or perhaps even many cases. I would suggest that in those cases of premature upgrading, that Delta accept that it was their error and provide an upgrade voucher to the displaced passenger for a future flight. Which passenger is open to debate. regardless, that seems to be a reasonable compromise since obviously you can't put two passengers in one seat.