I had a pax in UA I class to LHR and UA canceled the flight. They only offered to rebook him in Y, as "BA" will reject if they rebook him on their metal because it was originally an award reservation.
Well, this, IMHO is complete BS. Its not as BA doing a favor for UA, it's UA actually paying for BA for selling them a ticket and UA pays whatever they take from BA, NOT what the original UA ticket was. In fact I my self had a UA mileage ticket in business class and they rebooked me on BA F! Since J was sold out and everything was perfectly fine.
The Lufthansa group wasn't on that list either
But they DID allow free changes
Printing a boarding pass doesn't always mean ok to Travel. I once had a story were a boarding pass was printed but they were denied boarding as they had skipped a segment a few "days" prior but the website still allowed a BP to be printed
Well, you know what the case there was, the pax had a BP and was good to go but when the airline realized that he skipped a segament they just suspended the ticket. That is a valid reason.