I'm looking to get an upgrade on a flight from LAX to GRU. I see B fares available online, but when I call they want mileage and extra fees to upgrade. (I can see there is business class available on these flights too.)

I've upgraded B fares domestically but never internationally. When I've done it domestically I've just clicked the upgrade button and all is done. But are the rules different internationally? If you buy a B fare are you just first in line for an upgrade unless you pay the fees?

Any help understanding this would be appreciated!