I had trouble doing this. All flights out of PWM had been cancelled due to low cloud cover, and I was forced to ride the bus (on my dime) to BOS in the middle of the night, where United had set up a connecting itinerary back to SFO via ORD, much later in the day. I asked the ticket agent to put me on the standby list for the first nonstop of the day. He wasn't able to do it, nor could the first United Club agent make it happen. A second UC agent did something like a long sell, which took quite a while. Eventually I got my standby card and proceeded to the gate. I was the only standby passenger, and barely made it on this overbooked Monday morning flight. Finally, the gate agents had trouble reconciling the change and asked me to proceed on board without retaining a boarding pass stub.