I don't know if you can use exired upgrades; best thing to do is ask the ticket or gate agent.
Some of AS' flights on certain routes (ANC-SEA, LAX/SNA-SEA) tend to fill up pretty quickly. A rule of thunb is to make a reservation as soon as you know you are travelling and upgrade then. I've been fortunate enough to walk up to the check-in kiosk and find that there are upgradable seats available - this is rare, though.
You can check e-bay for upgrades. I've seen some go relatively cheap. If you have items to trade, go to Coupon Connection on this board (or e-mail me directly, I may have some to spare).