I have had difficulty with some itineraries when wanting to do a SDC. If I am trying to change routing between one hub to an other, for example taking a flight that connects through DTW rather than MSP or if I booked my paid F ticket using a combination of an e-credit and cash, those itineraries have needed to be changed by a rep. It just happened to me on this past Friday, I wanted to do an SDC from a DTW connection to an MSP connection later in the day. It was booked with e-credit and cash and I could not change it or even cancel it online, needed to call in to Diamond desk to change it. As a side note, the quoted call back time was 2.5 hours at about 8pm PDT. I sent in a complaint to Delta, as I have been doing quite a bit it seems lately. I got my canned response from Delta to my complaint about an hour before I got my call back from the DM desk. Agent took care of the SDC in about 3 minutes.