My partner and I recently were in HNL for vacation. We booked Y class tickets on theEWR - HNL nonstop and used our miles to upgrade which was confirmed at the time of booking in february. Our onward journey was fine but on the return, despite being checked in online the previous day, they downgraded me (and not my partner) 2 hours before the flight and in fact put me on the next days morning flight via SFO. On complaining at the airport, the said first was sold out but they did put me in coach. On this 10 hour flight with lie falt beds in first, it makes a difference. What would be my best recourse to get compensation apart from the miles - it doesnt seem fair that this can happen unilaterally. By the way, customer service on the phone and the agents at the airport could not figure out why this had happen - there was no reason or justification.
What a bummer. That is a very long flight to lose the upgrade, particularly when you were looking forward to the lie-flat. If I were to hazard a guess as to what happened, I would say "Federal Air Marshall" but that would just be a guess. When they need an F seat, they are going to ding the upgrades first, likely in order of frequent flyer status (i.e., lowest status first). So that's probably why you were the unlucky one.
As suggested already, your best recourse is a brief e-mail to United customer service stating that you were involuntarily downgraded HNL-EWR despite having a confirmed seat assignment in F.