I just wanna add my two cents here. I'm currently platinum medallion status with Delta. I use the SDC option a good amount of time. Once I'm at 24 hours before my current departure time, I just go to the "Change or Add Flights" option on either the Delta App or on delta.com. When you search for other flight options on the same day, the alternate flight options that are presented should show the cost or difference in price. The flight options that show 0 (zero) price difference are of course the ones available for SDC. Other flights could have a big price difference. I'm guessing you were asked to pay the price difference and the delta app asked for your credit card and security code info to process the charge. Sometimes a lot of flights are available for 0 price difference (SDC) or sometimes no flights available for the "free change." And of course your other option would have been stand by for the earlier flight. You probably had a very good chance of getting on the earlier flight via standby as a DM.