I don't have ff miles but I'm travelling from Charlotte, NC to San Francisco and am probably taking a UA/USAirways Airbus 320 non-stop. I'm going f class on the connection from San Francisco to Beijing, but am really wondering if I can purchase an upgrade at the departure gate or check-in in Charlotte? Is this a reasonable idea or am I dreaming? I don't fly very often...last time was last summer to London with my family, but I usually take a flight out of the country. Anything I can do to help myself other than hoping for a purchase opportunity?

Thanks...

betsy