I disagree with the opinion the OP was treated fairly. Regardless of what United normally does, there was intent to enter into an agreement, an agreement was reached, and consideration was exchanged.

In more equitable situations, one may refer to United's actions as a breach of contract. Even though the contract was verbal, and is hard to prove in court, it was a verbal contract. Very strong evidence to support this exists in the issuance of the FC boarding class and the customer taking the later flight.

I understand that it is not worth it to file suit, and United probably has some clause that says we are always right, and you are always wrong, but have a little love for the OP.

He had the boarding pass in his hand.

Personally, I am not thrilled if they change my seating assigment after I have the BP (without good reason) - I would be pretty unhappy if they downgraded me also. I can't wait to hear cries that he wasn't downgraded, but he was.