I had a slightly different experience...flew on AA2404 LAX-DFW-TPA last month, direct flight with a stop...I was able to request an upgrade on only the DFW-TPA segment. When the upgrade cleared, it showed my coach seat assignment on LAX-DFW and then the first seat assignment on DFW-TPA.

Only strange thing occured at checkin at LAX, the kiosk wouldn't allow me to print boarding passes and sent me to a "live" person. The ticketing agent said it was because I was in two separate seats for the same flight number (albeit different segments).

Interesting other note--prior to the upgrade clearning, my seat assignments, both in coach, were different for the LAX-DFW and DFW-TPA segments.