In late May, Delta Airlines changed its algorithm for upgrades on international flights, making it nearly impossible to clear mileage upgrades for ticket purchases that are made fewer than 14 days from the travel date. This largely unannounced adjustment changes the way Delta handles mileage upgrades to fill empty premium cabin seats. In the past, upgrade inventories would usually be available two weeks, one week or even just three days before a flight’s departure for higher economy fares in the M, B, and Y classes.

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