Don't think there is any risk of bump from J, even on a F ticket.
In short haul AY offers "seat guarantee" for plats, ie they will ticket a plat even if the flight is 100% full until 48 hours before takeoff. That could theoretically lead to bumping from J, but this guarantee does not exist in long haul.
I have however seen "oversold" J in longhaul where no one got bumped. They put pax on the jump seat instead, believe it or not...