I think this might have been discussed previously but I searched and didn't find it...
Lets say I'm flying from DEN-BOM on Lufthansa.
If I purchase an upgradable fare, can I use my Mileage Plus (UA) miles to upgrade to the next cabin?
What about with the other *A carriers, lets say I was flying to Bali... first on UA to SFO then SQ the rest of the way...?
The reason I ask is that I'm sure I could do that with NW/KL/CO, and I want to know if I can with *A.