IME this sort of a modification can be done at the ticket counter but is time consuming so get there early. It is also exacerbated by having the two airlines involved. Even if it is not raised at LAX I would suggest you point it out and try to get the AA folks at LAX to fix it. When you get to DFW you will likely be dealing with IB staff- who won't be able to change the AA booking and will need to get AA on the phone. Also by getting it addressed in LAX you can control the timing/when you arrive, whereas at DFW the duration of your layover is out of your control.