Originally Posted by
anabolism
Officially, SWUs can only be used on flights both operated by and sold (coded) as AA. There are anecdotal reports of gate agents allowing one to use an SWU to upgrade an AA-operated flight sold as a codeshare. But there is no way to use an SWU for a flight not operated by AA.
This is incorrect. It has been announced that with the merging of the programs both AA- and US-operated flights will be eligible for SWU upgrades. Whether that means it will include codeshares, i. e. AA-operated booked as US codeshare and vice versa (US-operated under AA codeshare) is unclear. The exact language is:
use systemwide upgrades on flights marketed and operated by American and US Airways