One of two things could happen: (a) you get a credit that can only be used for an AS/UA mix; (b) you get an unrestricted credit that can be used for any flight. Per the rules, the former is the correct outcome; however, I think the latter may be the more likely outcome. That said, I’ve seen reports of people being told “Sorry, your ticket can’t be moved to XX airline; you have to fly YY,” so if this would be an absolute non-starter for you, then you should consider a different set of flights.

It won’t be easy to tell the difference between the two outcomes until you actually attempt to use the residual credit for another flight.