This:
Originally Posted by
Perche
I recall reading that an upgrade from an AA ticket only gets you to a BA MCE equivalent.
doesn't have anything to do with redeeming an award ticket (it's also a very incomplete sentence regarding content).
I will assume that we are all talking about using
AA miles here (any discussion otherwise belongs in that airline's forum)
An award upgrade using AA miles on BA metal, first requires a full fare ticket. BA's WT+ isn't really the equivalent of AA MCE. BA considers WT+ a separate cabin, so an award upgrade from BA WT only gets you into WT+.
Next, when you use AA miles to book TATL, you have a number of options in terms of airlines. The majority of flights will be AA or BA, although there are also IB, AY, AB, and others. Whatever flight and cabin you book will be whatever flight and cabin you will be ticketed on. You cannot use AA miles to book premium economy because AA does not offer such a class.