A US codeshare flight operated by UA can be upgraded with 500-milers, CR-1s, and SWUs, but only at the airport on the day of departure. From the E-Upgrade terms and conditions at
here :
Valid Carrier: This upgrade certificate may be used only on United® or United Express® flights (only on multi-cabin aircraft) and not on flights operated by other airlines. Upgrade requests for flights marketed by other airlines but operated by United are only permitted at the airport on the day of travel, subject to availability.
This is discussed
here.