Use SWU on AC and then change FF# to another program for the flight?
 

Is it possible to book a flight on AC, use an SWU for an upgrade then change the FF# for the flight to another carriers program? I am more interested in accumulating miles on UA than AC but I've got all these SWU's are are going unused from AC. So I'd like to have the best of both worlds, but don't know if it's allowed

Yes it can be done.

I used to do it all the time when I had status on AC. Some agents may refuse and point you to the wording that you have to be an Prestige/Elite/Super Elite member to use the cert; just remind them you are but choosing to deposit the miles elsewhere.

Best place to do this is the MLL - never had an issue with it there at all.