I’ve done this a few times, but the opposite way - had a 6am departure LAX-JFK because that’s when I could use a cert or the D1 fare was half the cost of a later flight. I SDCd to the later 9am or 11am departure I wanted all along. Worked like a charm each time. Since it was 601am the day before that I called in, it was still over 24h before the flights I wanted so I think there were more seats available as all the upgrades hadn’t processed yet.