Originally Posted by CThomp79

FWIW, flying SFO-JFK and back in late July, I was successful making a walk-up upgrade both ways. I was pleasantly surprised to find a little over half the F cabin available for upgrade when I checked in. For this route and during a busier travel season I had expected my chances to be lower. Given this experience, I would expect availablility SFO-IAD to be as good or better, though as others have said it may vary significantly.

On a sidenote, as a relatively frequent UA flyer, it is interesting to consider the difference in F availability on VX since frequent flyer status is not a factor. In fact, dare I say, it's refreshing not to have the "gate lice" effect of 1K/GS/*P pax checking to see if their upgrade cleared.