Originally Posted by
makfan
SWUs cannot be used on codeshares. In fact, be careful about buying AA ticketed/US operated and vice-versa. Generally you need operated and ticketed to be the same carrier to use a SWU.
This is correct, as is your (the airlines have announced it as well) advice to avoid codeshares for now if one wants upgrade ease.
SWUS are only usable on AA operated flights, and generally on AA marketed flights as well. They ONLY are usable on non-AA flights when one has been granted its use in an AA flight but one is moved to a oneworld partner because of IROPS.
E.g. I've kept SWU use when SFO-ORD went IROPS due to ORD ground holds and I'd have missed my ongoing flight to Europe; our AA flights had been upgraded with SWU. AA moved us to BA SFO-LHR in C.