To be honest,​​​​ I've often asked but never gotten a reply where FB has upgrade availability. Presumably, they're seats made available to FB for miles purchase/upgrade, and like other fare buckets, they get made available at various times as the revenue managers adjust to maximize income and fill the plane. So in theory, at various gates: D-180, -120, -90, -30, -14, or whatever AF/KL use, the bucket of fares for FB upgrades could get refilled. But it probably won't.
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At the check-in, you can ask if they're selling upgrades, and those transactions can be done in miles. Good luck, since travel this summer features sold-out premium cabins.
​​​​​​On the online check-in, when you select a seat, you can upgrade by selecting a premium cabin seat and paying what it charges.