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.
Again, or rather, as usual, the tone in this thread has started to lean toward blaming the customer. If you called your bank and because of some error they waived a fee but it showed up on your monthly statement anyway, because she didn't put the correct code in their system, would you be angry or rather accepting that she didn't hit F5-B-27-Y-Y-Y? In other words, should it be the customer's job to know the procedures behind customer service practices? Yes the agent should have justified the upgrade with points or certs, but
the OP is not responsible for UA policy, its training program, procedures, nor its quality control.
IMO, a representative of United offered him an upgrade, sent him the confirmation of the upgrade and that should be that. Complain to customer service, do not let this go.