I have found DL to 'hold' up-grades for the day of departure. My tickets are usually purchased the day of, or day before departure, and I am 100% up-grades this year except 2 IOPS, and that is over 100 mainline segments.
One of the many reasons I left US was their change of policy to clear all possible UG the morning of the flight. This left a full fare Chairman Preffered in a middle seat in the back. Not cool. If DL did really change the policy, I am gone with my high yield fares.