It's vague on the web site...
Within or between the continental U.S., Alaska, and Canada
From Full Coach Fare 5,000
From Select Fares1 10,000
1Applies to select published fares and specific fare classes. Please check with a Delta reservations representative for eligible fares applicable to your itinerary.
but L, U, and T fares are not upgradeable domestically with miles or PMUs (Platinum Medallion Upgrade certs). You are eligible for complementary upgrades at the gate (T fares don't clear for upgrade until just before departure.)
You might be able to buy up your fare to a K fare and use miles to upgrade.
FYI, international fares must be Y,B, or M to use miles for upgrades (no medallion status is required).
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