Originally Posted by
niji248
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
When you get a comp upgrade to C+, it should show the fare class as "SU" on your itin, but your underlying fare does not change and is still a Main fare (there is no fare re-issue). Comp upgrades to FC work the same way, but fare class will show as "OU". Again, your underlying fare is still a Main fare (if that is what you originally bought) and you don't have the ability to SDC to another FC seat on a different flight since you don't have a purchased FC fare. In the same way, a comp upgrade to C+ doesn't confer the ability to SDC to another C+/Main seat just because they are still available. The Main fare class heirarchy is Y/B/M/H/Q/K/L/U/T/X/V, and so it is not unusual for there to be no X inventory close-in to a flight since it is one of the lowest Main fare classes.
The SU and OU inventory classes are what controls when seats are released for comp upgrades. They generally do not hold back any C+ seats from the SU bucket class and PM/DMs will pretty much always get comp upgrades to C+ at booking as long as there are still seats available. OU upgrade class is a different story and they will generally hold back some FC seats from the OU bucket until they are finally released at the gate around 30 minutes before departure.