From LAX to Bali "buy economy seats than upgrade later" is a way too optimistic and impractical. For a start no USA based airline fly to Bali. Your AA miles (status) may help get an upgrade on an AA operated flight. But many others will be trying for the same upgrades.

Generally you need ff miles (and sometimes cash) and/or an upgrade instrument from the airline you are flying with to upgrade, if available. Airline ff status can help a lot to get higher up the upgrade list. Now days you often need an expensive base economy fares as a starting point.

Cross airline upgrades are not common. AA/BA/IB have a restricted upgrade system. Some Star Alliance airlines have a cross airline upgrade system. But the t&c’s make these useless for most people.

Upgrades on non USA airlines operate very differently to upgrades on USA airlines. As for upgrades with airlines based out of the USA always expect to fly in the class you buy. It is only USA based airlines that give out upgrades like confetti for free (by status or no cost upgrade instruments) or with miles-cash co pay. There is an expectation by many USA frequent flyers they deserve a (free) upgrade.

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