No reroute is allowed on the ticket rules, as already mentioned. Your only hope is an irregular ops reroute, but CX rarely cancels flights or has a misconnect. AA has to reissue the ticket and won't do so on this type of award. CX won't get paid a penny if they do wind up flying you trans-pacific. All you can do is scrap the award and buy a one-way ticket (probably a lot cheaper from BKK than from HKG or NRT). In any case a 20 hour layover in Narita City is rather nice this time of the year, lots to see and quite enjoyable. Next time get the AA all-airlines award if you need this kind of flexibility; the OW award costs you this flexibility in return for its other benefits.