Q: I bought an economy fare on Delta from Boston to Los Angeles, hoping to upgrade with miles to business or first class using 5000 or 10,000 miles. But then I discovered that my "fare class" was too low to qualify for a miles upgrade. Do all airlines have this restriction?

A: Delta only allows SkyMiles award upgrades "
from select fares." They don't specify what those fare classes are on their Web site, but the 5000-mile upgrades are only applicable to full fare coach domestic tickets and the 10,000-mile upgrades to slightly less expensive fare classes. They cannot be used on most heavily discounted fares, unless you are a Medallion member (a very frequent flyer), in which case many of the cheaper fares are eligible for upgrades for free, anywhere from one to five days in advance of travel depending on your exact status in the SkyMiles program. United, in contrast, is better in this regard. You can upgrade
S, K, L and T class fares, which are some of their discounted fares, by redeeming 15000 miles on a coach domestic fare. But many airlines don't allow their very cheapest seats (including, sometimes, those bought on
Hotwire or
Priceline) to be upgraded under any circumstances, so it's a good idea to check with your airline before purchasing in order to avoid disappointment.

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