Isn't "stole" a bit harsh? The agents on the NRT flight hadn't handed out the remaining upgrades yet. It's not like the OP took a C seat from a customer who already had a seat assignment.

UA was the party that took a C seat -- from the OP on the ICN flight -- so that they could give it to someone who was willing to pay more for it. That's not stealing either -- it was just very uncool, and produced a bad customer experience that eroded the OP's trust in the airline (and my trust too, for that matter).

If UA starts taking things away after we've paid for them (even with non-cash currency like a SWU), how can they expect to keep our business?