It seems there are five different YUPs for JFK/LAX 3-class transcons. Three of them upg into D or I (Biz) and two into P (FC) only when a r/t fare is purchased:

YUPPRZN AA P Round-Trip
Y3CUPZNR AA I One-Way
YUPPRMZ AA P Round-Trip
Y3CUPMZ AA I One-Way
Y263CUP AA D One-Way

In looking at the fare rules for the YUPPRZN that upgs into P, it seems to be very specific about the dates and flights. I'll leave the interpretation to others.

FLT APPL
IF TRAVEL FROM THE POINT OF ORIGIN COMMENCES BETWEEN 27AUG AND 27AUG05, THEN APPLIES TO ANY FLIGHT EXCEPT AA 15 AND IF TRAVEL FROM THE POINT OF ORIGIN COMMENCES BETWEEN 3SEP AND 3SEP05, THEN APPLIES TO ANY FLIGHT EXCEPT AA 15 AND IF TRAVEL FROM THE POINT OF ORIGIN COMMENCES BETWEEN 10SEP AND 10SEP05, THEN APPLIES TO ANY FLIGHT EXCEPT AA 15 ON SAT AND IF TRAVEL FROM THE POINT OF ORIGIN COMMENCES BETWEEN 17SEP AND 17SEP05, THEN APPLIES TO ANY FLIGHT EXCEPT AA 15 AND IF TRAVEL FROM THE POINT OF ORIGIN COMMENCES BETWEEN 24SEP AND 24SEP05, THEN APPLIES TO ANY FLIGHT EXCEPT AA 15 AND IF TRAVEL FROM THE POINT OF ORIGIN COMMENCES BETWEEN 1OCT AND 1OCT05, THEN APPLIES TO ANY FLIGHT EXCEPT AA 15 AND IF TRAVEL FROM THE POINT OF ORIGIN COMMENCES ON/AFTER 3OCT05, THEN APPLIES TO ANY FLIGHT EXCEPT AA 15/20 AND APPLIES TO ANY FLIGHT EXCEPT AA 1-4/10/16/AA 18-19/AA 21-22/30/AA 32-34/40/59/85/AA 117-118/133/AA 180-181/185/ 188/193/201/242/245.


Edited to add: Just did some spot checking and there is now a P class listed on the LGA-DFW flights and the YUP fare books into this P class. Virtually the same rules as above for LGA-DFW. Looks weird to see "P" for an MD80 flight. There is still YUPs booking into D, I, A class for some other routes I checked (i.e. NYC-HNL)