Originally Posted by
Robert Leach
Hawaii has different mileage upgrade rules than any other market in the Delta system.
Whereas all other mileage upgrade awards, whether domestic or international, can only be used with M fares or above, to Hawaii the requirement is a K fare or above.
In other words, you cannot upgrade L, U, or T fares to Hawaii, but a K fare (or above) would work.
In case others are wondering why (which I was a few months ago) it's b/c Delta doesn't offer free medallion upgrades on Hawaii segments...