vargenson , it is not necessary to have an AAdvantage number or even a reservation just to view the seat map for your LAX-LAS flight. Just go to aa.com or the AA app and pretend that you're trying to make a new booking for the same flight. When the flight comes up, click on the "Seats" link, and the seat map should be displayed.

If you decide that you do, in fact, want to change your seat, you can call AA, chat with them via the website or app, or create an AAdvantage account.

Edited to add: The LAX-LAS flight is scheduled to take all of 75 minutes; actual time airborne is about 45 minutes. Unless you are prepared to pay extra for a "Preferred" standard Coach seat or for a "Main Cabin Extra" (extra-leg-room) Coach seat, I would not fret too much about your seat choice for such a short flight.