Your upgrade will be treated as if you had just walked up and bought your new ticket, and asked for an upgrade -- if there's availability, you'll get it, and if not, you'll be waitlisted.

When there are many volunteers, some gate agents prefer to take those traveling without connections. Just depends on the gate agent, but I'd say this happens less than 25% of the time in my experience.

edited to answer your second question: Accepted wisdom on FT is that they're supposed to give you the higher mileage of the two routings. But I've talked with UA agents, and the reality is that a bump rerouting is considered "voluntary," and they are only required to give you what you actually flew -- but as a courtesy, they will often give you the miles for the original routing you bought if you ask nicely.

[This message has been edited by pynchonesque (edited 02-15-2002).]