Does anyone know what happens in the following scenario? Suppose I buy an economy ticket for an AF/KL return flight from AF/KL, then call Delta to use a GUC on the outbound, and then want to change the date of the return flight. From what I understand when I apply the GUC, Delta will reissue the ticket themselves. So at that point do I need to call Delta to change the date of the return, or AF/KL?