Thank you, dvs7310!
That was something I wasn’t sure about if it’s possible to use miles for at-the-airport upgrades, and if they can be non-native *A miles.
Apparently the answers are yes and no.
Thank you for the tip about the Marriot miles! That would be a good option. I have some SPG points, which are compatible with Marriot points AFAIK, but they will not be enough. So I’ll have to wait until after the first leg of the journey.