Q. Are all economy-class fares, including the 21-or 14-day advance purchase ones, eligible for upgrading using frequent flyer (air) miles?

A. Theoretically, at least on most US-based airlines, the answer is yes. But on other airlines, the answer is no. Airfarewatchdog has always maintained that one of the “highest-value” ways to spend frequent flyer miles is to buy a super cheap economy class domestic fare and upgrade it (usually with 15,000 miles each way) to a super expensive business or first class fare, rather than spending 25,000 or even 50,000 miles on a cheap economy class seat. However, some airlines, most notably the world’s largest, Delta, allow upgrades only from their most expensive economy class fares, assuming there are even upgradeable seats available. United, American, and Continental, however, allow upgrades from almost all economy class fares, but they now require co-pays starting at $50 each way on domestic flights up to $700 each way on international flights (Delta does not require co-pays). These hefty co-pays on international flights sometimes make upgrading with miles a bad bargain. Add the hassle factor of finding flights that have upgradeable seats, and it’s no wonder that more consumers are using cash-back credit cards rather than cards that earn frequent flyer miles.

This article provides further information on which fare classes are upgradeable for the major US-based airlines.