I need to get from lax to cdg and then cdg to lax. I’m thinking of booking premium economy tickets then use flying blue miles to upgrade.

I want to fly delta premium select as I heard they are better than AF. Can I use FB to upgrade on delta? Or it has to be AF? Note that I’ll also be booking with AF not delta.

or I can do AF, but I’m worried that AF is not as nice as delta and if I book in AF and cannot upgrade then I’m stuck with AF.