Well, after 45 minutes with the DL agent, finally had to get a supervisor on and they did "something" (not sure what) to get the ticket to process. The original DL agent said that their system just wouldn't let them change the itinerary as it showed not for sale (inventory). I don't know. Very odd. I'm not familiar with DL systems or why it wouldn't let them see a flight that DL.COM could see. Even though the agent could see the DL.com segment, neither she, nor the support desk could get it to show up as "sellable" inventory.