Originally Posted by flyerCO

I've split off a companion after they've been upgraded. They did not lose the upgrade. However if something then occurs to require rebooking they get put back in Y. There's nothing that says once you split, an already upgraded companion must go back to Y. Only time that is true is if you change to no longer flying on the same flight as them. Of course I can bet if 100 of us called, we'll get 100 different answers on this.