Hmmm.

First of all, sorry about the frustrating experience. Not fun at all, and I feel your pain.

It seems to me that if UA sold you a ticket, charged your credit card, took your miles and gave you the inbound upgrades, then it's too late for them to reneg on the deal. They made you an offer, you took the offer, and if a mistake was made, it's their problem not yours. Might be too late, but I'd escalate this one up the ladder until you find a sympathetic CSR. (Do you have status?)

I was once in a similar, but slightly different situation (you can PM me if you'd like the details). Basically, UA sold me something, later claimed a mistake was made, and wanted to undo the entire thing. My argument, carried through two levels of CS supervisors was (a) I was offered a certain ticket at a certain price, (b) I paid that price, and (c) if mistakes were made, it was their problem, not mine. I was firm but persistent, and eventually prevailed (with some minor concessions on my part).

Good luck!