Another problem with DL upgrade certificates, unless you happen to be Gold or Platinum Medallion, is that you can't use them until 24 hours before flight time. And that's on each leg. If I'm leaving DC at 12:01 on Tuesday to Atlanta, I can't call until 12:01 on Monday; if my return is at 12:01 on Thursday, I can't call until 12:01 on Wednesday. Even if I paid for my upgrades.