Welcome to Flyertalk,
lawest!
AA does not fly to Hong Kong themselves; You could fly to Tokyo on AA and upgrade as far as Tokyo while continueing on an AA codeshare (operated by JAL) to Bangkok.
Is is worth purchasing a more expensive ticket?
This depends how much you value business class (personal) and how much you dislike coach and how much $ you have.
As for upgrade changes, if you book midweek you have a good change of being upgraded. You could also use Expertflyer.com to see if AA has any flights where seats could be confirmed for upgrades right away. I did a little checking and see that on this route (LAX-Tokyo) upgrades would have to be waitlisted for most every date.