Nonmosaics should still be able to on a Blue fare for $75 starting at midnight on the day of travel. Mosaics can do it for free starting 24 hours before the ticketed flight.

Yeah I've noticed the inability to do the SDC online recently, and have needed to call the last few times (all of which the calls have been under 5 minutes total, so not a huge inconvenience). One agent said they had a fix coming soon, but who knows if that's true.