FYI I tried doing this last week and failed. I was scheduled on a flight between midnight and 1am and tried to switch to a flight a few hours earlier on the calendar day before. Called the EXP line three times to try to switch to a flight with available E inventory, they all said no (two of them put me on an extensive hold to go research further.) I even tried doing it at the airport at the check in counters the day of and also got rejected. It's possible had I gone through security and found a friendly gate agent maybe they would have put me on standby, but I wasn't wanting to get on the earlier flight so badly that I was willing to sit around at the airport and wait an extra few hours if I failed. I was also already upgraded on my existing itin and knew if I was standby and not SDC the chances of being upgraded if someone did help me change were a lot lower since all confirmed passengers on the standby list would get upgraded first.

Anyone know figure out what the official policy is?