Here is the "canned" response I received a few weeks back when an almost identical situation happened to me. I was taken off of a ATL-OKC flight where I was confirmed Y and I had already cleared F on the upgrade. My original SAV-ATL flight got canceled, I rebooked to an earlier flight SAV-ATL, and arrived in ATL with 4 hours to kill before my connection. Imagine my surprise when I showed up at the boarding gate for ATL-OKC to find I was not on the flight! I cleared standby and sat in a middle seat.
The response from DL was classic.
Your time is valuable, and operating on schedule is equally important to us. However, when a mechanical problem is found, we must restore the aircraft to company and federal airworthiness standards. Safety is the number one consideration in the operation of our flights.
I'm sorry we could not confirm your upgrade at this time. We monitor every flight carefully to ensure upgrades are provided to our most loyal members. At the same time, we need to be sure that other customers are well positioned to purchase these premium seats in order to maintain a financially viable airline.
Again, thank you for writing. Please accept our apology for the circumstances you described. Your comments are appreciated since this is
one of the best ways we have of knowing which areas need additional
attention. We hope you will continue to make Delta your airline of
choice.