Originally Posted by
star_world
You certainly can upgrade on the majority of *A airlines if you're on a Y/B fare. This is a standard *A benefit, and there is no co-pay - just miles.
Of course, but it's worth noting that you would need to redeem a separate award for the intra-European segment. And if you made it to MUC or FRA or even ZRH on CO with a BF upgrade, then the leg to BUD will be only about an hour, and hardly worth using the miles to upgrade, even though it's only 10k each way.
It might be worth it on the return, depending on your *A elite status and thus your lounge access and baggage allowance when checking in at BUD.