When you buy the ticket, and go to the website/app to check your booking, you will see an invitation to upgrade with miles. If you click on that, it will always lead to a page that says "give FB a phone call". When you call them, they will tell you an upgrade is not possible.
in theory, it is, provided they have available seats in that upgrade-friendly (Z?) fare class. In practice, I've never succeeded, and I had a habit of booking strategically into W for the J upgrade (when available. And let me just say, 12 hours in those PE seats ain't fun, nor is it habit-forming, and I've done that a few times now).