You can do a same day confirm with in three hours of the flight you would like to change to at no charge if you are a gm or pm. I have done this many times when a meeting ends early and there is a flight earlier then my original flight. For example if you are booked on a 6:00 pm fight and you are able to leave on a 1:00 pm flight you can book the 1:00 pm any time after 10:00 am.
I hope this helps you. This is one of the best perks delta offers.