Your RTW tickets are only valid for the discount class for which they're issued, so availability of full-fare seats won't do you any good. You may implore the airline to make more discount seats available on the flight you need - sometimes that works, sometimes not. AA is fairly stingy.

Alternately, is there anything keeping you re-routing NRT-HKG-SFO on Cathay instead of taking the AA flight? There will be a change fee of USD125. That assumes Cathay has seats, of course.

Please say the class and type of tickets or more help.