Yes, we did this in January from MCO to SFO. The night before we decided that we wanted to get home earlier (don't ask me why) so we called Delta and checked it out. They said that there are no guarantees but that it "looked good" (there were lots of empty seats on the flights that we wanted). She gave us some flight times and we showed up early, never had any problem at all. We had a layover with a change in planes in Dallas and again had no trouble. They didn't charge us anything. Of course, you run the risk of a plane being full, but its my understanding that if there is a seat same-day, you can have it at no charge.
Hope this helped!