If you're earning UA miles on your itinerary, you can purchase 500-mile upgrades (PT500's) for the domestic portion of your itinerary at check-in on the day of your flight. You will then be WL. If you are on the A320, there are 8FC seats and you will likely not get seated. If your flight is the A321, there are 26 FC seats and you will have a shot of getting it at the gate. KEEP ASKING. Just remember, you will have to have the 500 milers IN HAND. In most instances, you cannot purchase them at the gate - you'll have to do it at ticketing. And, if you don't clear, the UG's are yours to cherish and keep. Just a note - you have to be specific and ask to purchase PT500's. They came about after US and UA started the partnership, but many TA's don't even know they exist. You will need 5 ($250) for the ow SFO-PHL.

As far the the International portion, it's not good news. There is no provision for persons earning UA miles, traveling on a coach ticket, to upgrade flights to Europe. If you were earning US miles, there are several ways to get hold of a SA upgrade to be used day of departure; and you can generally eek out one seat when WL'd But, earning UA miles on US metal, you can't upgrade using miles, and you can't buy int'l upgrades.

If you're on a higher coach fare, you may get lucky at ticketing in SFO with one of the great former PSA agents who know how to work the system, but the airline is very tough with Envoy upgrades. You can also ask at the PHL gate.

The key is to ask, ask, ask. You never know who you'll get who might like to bend the company's rules with everything that's going on.