I pulled up the fare rules from aa.com for a SFO-LAX round trip leaving tomorrow morning and returning that evening. There are two fare basis codes, one starting with L and one Y. This is the L rules:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">... IV. AFTER DEPARTURE - WHEN THERE IS NO CHANGE TO ORIGIN/DESTINATION OR STOPOVERS. --- A. CHANGES MAY BE MADE ON/PRIOR TO TICKETED DEPARTURE DATE OF EACH FLIGHT FOR THE CHANGE FEE WITHOUT REGARD TO ADVANCE RSVN OR DAY/TIME REQUIREMENTS PROVIDED THE CHANGE MEETS ALL OTHER FARE RULES. ...</font>
(emphasis added)
What AA does not seem to understand is that a round trip fare basis code is *more* restricted than a one way fare basis. That is, you cannot use half of a round trip fare basis as a real one way trip. But we aren't talking about one way trips, we are talking about a simple round trip.
The fare rules clearly allow you to change the return portion of your round trip for $100.