Thanks. And AA doesn't put the critical sentence "The segment must be converted from a codeshare to a prime flight" on the publicly-facing partner-upgrade page because the conversion usually just works and the passenger doesn't even need to know it happened. And they cover the can't-convert cases by listing the partner classes they can. Except that they are months late (typical!) in dropping B from that list.

Got it. I can finally stop my weeks-long head-scratching over claims that "PE won't work because you have to convert".

Hmm, so, while the PE problem will (maybe?) go away when AA fully rolls out PE, the B disconnect presumably will not.