There are 18 open first class seats on the seatmap of a mid-day 757 (the flight shows F9).
It’s within the 1K upgrade window. It’s within the Premier Executive upgrade window. There were no upgrade seats available before the 1K window, and there still aren’t. Do they really expect to sell possibly 75% of the cabin in the next 3 days?
Sometime around August of last year United just about stopped making advance upgrade inventory available on its flighs between Washington-Dulles and California. There’s the occasional flight with a solo upgrade seat, the occasional redeye with more than one, but advance upgrades on transcons seem to be a thing of the past to and from DC.
Connecting flights, e.g. Dulles or National to the West Coast via Chicago or Denver, are much more confirmable in advance.