as a 2P, how were you clearing an upgrade 4 months in advance? Unless it was with miles, it sounds like a mistake on the agent's part to give you the upgrade, and although you received a confirmation, it was still a mistake. And then in some nightly computer sweep, it was cleared out as an error because your record had nothing to back up your seat in F. If this is the case, I myself wouldn't push the issue, even if I had the confirmation, because I received something I shouldn't have, and it got taken away, so no harm done (or at least I am not entitled to complain).

If it was an upgrade with miles then you might have grounds. But it doesn't sound like that's the case from your description -- sounds like you wanted to keep something that you were surprised to get by accident.