Delta domestic upgrading policies:

To use miles: Must be a Y/B/M

To use PMUs: Must be a Y/B/M/H/Q/K

To use a complimentary Medallion (V/X inventory upgrade) on yourself, or a companion that is Medallion: Y/B/M/H/Q/K/L/U/T

To use a complimentary Medallion (V/X inventory upgrade) on a companion that is traveling with you: the companion must be on a Y/B/M/H/Q/K fare (the Medallion can be on a Y/B/M/H/Q/K/L/U/T fare)

To do a paid gate upgrade (for nonelites): must be on a Y/B/M/H/Q/K fare.

Only way for L/U/T fares to be upgraded is via complimentary Medallion upgrades.

If the OP is on a T fare, the only way to get upgraded would be via his NW Platinum status.