On a few occasions, when traveling via DAL, I have been moved to an earlier flight for the first leg of trip but still connected to the same flight for second leg of trip. My motivation was to put me at DAL where the lunchtime food selection was better, though with a longer layover. If they had said no, no big deal to me, but hopefully the gate agents understand that once the flight leaves the gate those empty seats are lost for any revenue generation purpose. Putting me on the earlier flight opened space for possible sale on the later flight, which seems like a good business decision. Understandably, if it meant me getting to destination earlier, all the rules about fare changes, etc., would come into play. As said above, it never hurts to ask.