It is a round trip.

Generally speaking, only an endorsable fare (i.e. Full Y) would work, since most dicount fares are limited to the carrier publishing the tariff.

However, it the original carrier is unable to carry the passenger, they are required to get the person to the destination, even if that means another carrier in another class of service.

So, an AA EXP could fly outbound, upgraded by reason of status. Then on the way back, if AA cancels their flights, they would reroute with whoever is available, which might (gasp) be UA.