I was flying SFO - FRA recently. Upgraded my E+ subscription from domestic only to include global routes. At some point prior to the flight I was able to pick an E+ seat (E+ seats had been fully booked prior). Day of flight I had a boarding pass for the E+ seat.

Shortly before boarding, gate agent calls me up and says that there's been a problem and they need to move me. So they moved me to an E- seat. No reason was given, except for something about a mistake and someone else getting the seat. Since the plane was already 3.5 hours delayed and at risk of crew going over time, I didn't push. But I figured I should be entitled to a refund of the difference between the seat I got and the one I had. So I submitted a refund req and United says "we don't refund E+ subs". I am not asking for a refund of E+ sub; I am asking for compensation for an involuntary downgrade from a seat I had technically paid for.