Delta.com seems to spell it out.....
Example: If you are scheduled on a 9:00am flight and would like to take a 7:00pm flight on the same day, you can request the same-day confirm option as early as 4:00pm. If you request a change prior to 3 hours of the flight departure time, the standard rules of your fare will apply.
http://www.delta.com/traveling_check...nges/index.jsp