If the business cabin is full, then I assume they'll upgrade some passengers for free to the F cabin, most likely based on status. I'm flying on a flight in a couple months that had the option to purchase Royal Business seats, but I declined. I figure I'll wait till check-in and see if I automatically get a seat in F or not. If not, I'll decide at that time if it's worth it to pay then for an upgraded seat. Not sure though if it can be purchased at check-in or not.