Originally Posted by
MSPeconomist
Is it correct that there are only two levels of APD, namely coach (including basic economy and coach seats with extra legroom) and all premium cabins (premium economy, business class, and IFC) on longhaul flights? Specifically, if I've already paid APD for a flight departing LHR in Premium Economy and then I'm upgraded to business class, I don't owe anything more, right?
Assuming that you're traveling on AA, DL, or UA, and not BA, you won't owe anything more. You are correctly understanding the APD split; you can confirm this by doing an award search at aa.com, and seeing that the APD for Premium Economy, Business, and First is identical.
If you were traveling on BA, the carrier surcharge in Business would be higher than the carrier surcharge in Premium Economy.