I was trying to do the opposite of moving from a 4:45pm flight to the redeye at 12:40. I was proactive and called to see if the same day change would be available (I prefer to do that then standby so I can get a seat). I was told yes. I called that morning around 830 and was told I'd have to call within 12 hours of the new flight. I called in at 1pm and she tried to do it but it kept giving her an error, another agent got involved and found the issue that the new flight was 1 day later than the old and thus no same day change available. The flight was wide open so they did accomodate me since I had been told 3 times already that it would be fine but also made it clear that same day means same calendar day.