Same Day Change on a red-eye
 

I recall reading somewhere that SDCs can only be done after midnight on the same calendar day.

I want to SDC my connections on a 12:50am departure, does that mean I have to check-in last minute (after midnight) so my bags are tagged correctly?
What if the agent doesn't figure out how to do it in the 10 minutes between midnight and baggage cut off?

I am not SE so no concierge and assuming I don't get the option I want at OLCI.

I reached out to AC and was told that the SDC window opens when airport check-in (not OLCI) is available (usually 4 hours before departure I think).

Last week I did a SDC at 9pm the evening prior for flights the next day (changed from 3pm to 7pm departure). I did this at the ticketing counter at my departure airport and there were no issues or questions asked -- I thought the "same day" referred to 24 hours prior to departure? Although connections complicate things so maybe that is the issue, although it is still inconsistent with what AC has told you.

I have never been able to do a SDC prior to midnight.

That's what I thought but they said that if airport check-in opens before midnight (i.e. 12:50am departure) then they allow it.

I have it in writing in case airport staff give me a hard time.

I phoned in on this issue to get clarification before buying a ticket and was assured that on those post-midnight redeye departures you can SDC to flights during the daytime the prior day.