<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Plato90s:
I think the YUP fare can still be upgraded into X-class, via the same method as usual, even if there is no A-class inventory available at the time of booking.</font>
I think that is right (in which case the upgrade would be free). On the three-class JFK-LAX flights, however, the upgrade would be to R (iirc), not X. Whatever the inventory, you end up in business class, not first class.