I just did a "SDC" on a Comfort fare.

It's only 2335 departure city time. I was booked on the YVR-YYZ red-eye, and changed to an earlier one. So it no longer needs to be "day of departure".

Also, I was confirmed in R on the original flight. There was absolutely no mention of a cabin change when I did the SDC, but I ended up back in K. OLCI started to offer me an upgrade price.

Me, being me, had ensured there was R space on the target flight before doing this via OLCI, so I was able to go back into the eUp site and upgrade myself.

But SDC seems to work once your flight is available for check-in, rather than "same day".

Then when I checked in for the new flight, I picked a new seat, and I immediately got a text and email telling me my assigned seat has changed.

Also, based on app notifications, most screens in the OLCI process seems to update the PNR. Some change seats, some add SSRs, etc, but they all seem to make modifications worthy of sending me a notification.