Not sure if this is new, but I have an award booking today and the normal "Change Flight" option at check-in on website does not show any SDC options. However, if go through "Change or Add" flights option on website which normally does not work for SDC, I'm seeing a number of alternate SDC options for zero additional miles in same original NX booking class (the NX award I'm on has a 30 day AP). The App "Same-Day Change" option on itin shows the same SDC options. Not sure if this is because awards are handled differently for SDC or if there has been some change on website.