As earlier posters have suggested, many airlines still allow domestic and international upgrades on cheapest fares. AA is one of the largest carriers that is a good example. I am, however, not aware of 20,000 mile upgrades for transcon on AA, CO or any other major carrier. Availability is a different issue altogether.

I fly AA a lot - "A" inventory on transcon routes is usually hard to find these days on convenient schedules (since there are paying pax for YUP fares). EXPs do get upgraded almost all the time but anyone with a status less than an EXP has to face a lot of uncertainity. Sticker upgrades do work for PLATs but on Fri and Sun/Mon and other busy flts EXPs take all the remaining upgrades.