Most likely OP's PNR already has annotations about the interactions will different agents, so it might not help when calling back. However, I don't see why DL cannot rebook if they issued the ticket. I've had partner schedule changes in the past that have been rebooked without a hitch. As flyerCO mentioned upthread, I would try contacting DL again, explaining nicely the situation and the expected outcome (Full itinerary proposed) and if they refuse, ask to be transferred to the JV Desk, who might help in this situation. I've never seen partner award availability an issue when rebooking due to schedule change.