I have to admit I've never did a SDC on-line, I've always called, so if you want to do it on line I would...
1) not check in 24 hours in advance
2) after 10 pm pull up the res and click "change"
3) if I can't figure it out, then call?
Since our original flight is 5-something a.m. I hate having to be up after 10 p.m. to do this, if I can't make the change I have to be up around 3 to make the flight, ugh.