Originally Posted by RealHJ

I've had AA, UA and DL all put me on a revenue fare code with another airline when an award flight has been cancelled by them or by another airline. In case of UA it was DL in full F (that was surprisingly easy, I just asked for that and instead of two connecting UA flights that I was originally rebooked on, and I got on a much better direct DL flight, that only had one seat at full F open at the time, that UA was able to get for me in under 10 minutes) and in case of DL it was UA in P, while in case of AA it was AY in full Y.

So US here really should put you in a revenue fare class on an alternate flight of their own, another Star Alliance airline, or even a non alliance airline. It is up to them to accommodate you, and if there is no award space, they should put you in a revenue fare class then on any other airline that can get you where you need to go, when you need to go (the nice side effect of which then is that you earn full miles and qualification miles on that segment, e.g. on my UA award flight I earned full Delta miles + 150% MQM bonus, and likewise on my DL award flight I earned UA miles and also the P class bonus on that). Most airlines have such "other airlines accepting their tickets" in case of cancellation set-up, as they do it for each other from time to time as needed, so I am pretty sure that they don't actually pay the other airline full F/J/Y fare or what not that the fare class comes with (and likely nothing, it's probably just on a mutual barter basis I'd think).