So the day of travel, today, X inventory became available and I managed to do SDC, but now again the app is bugging out and couldn't generate a boarding pass after several "successful check-ins". I bought main cabin X originally, and was automatically upgraded to Comfort Plus, but the ticket remains in fare X. I didn't know Delta has this fare class restriction regarding to SDC, because AA has been pretty easy. In AA, you can do same day change as long as it's within the same cabin, so if you're even upgraded from coach to business on domestic transcon, if there's business class available you can still come in same day change to R/C/J class