They already overbook J. I think that it's rare to see any flight - or at least any route - unavailable to purchase in J more than 24 hours before departure.[I do recall someone posting here about the frequent lack of J seats on a particular AF flight from CAI to CDG several months in advance, though!]

As such, in 99.9999% of cases, you can buy a full-fare J ticket up to 24 hours before departure. And you can be sure you won't be the person bumped, if it comes to that.