I booked a flight SFO=>DFW=>MSY. It is same flight number all the way, only a stopover at DFW. It appears in my reservation as 2 flights, but the system let me select Only Same seat from the Overlapping Availability of the 2 segments.

It turns out there are much better seats for the SFO=>DFW segment. I called Platinum desk, but the agent told me she can not do it.

Does anybody have experience with such situation? is it possible to get diff seats? is there a Special name to this procedure that I have to tell the agent?
Thanks in advance.

Actually... if really not possible... I probably can ask an agent to cancel both seat assignments, give me the exit row for the SFO=>DFW and leave me w/o a seat assignment for DFW=>MSY