Waitlist what you want while you have the required points then book the confirmed flight (which then takes the points). Waitlist a few days if you have the flexibility. Then if/when one of the waitlisted opens up I believe you can just cancel the confirmed J and take the waitlisted F. The points come back pretty quickly when cancelling and I think you have at least a day to accept waitlist so all should work out fine. Maybe they can do it all in one go if you call? I haven't done the last part so not sure on those specifics.
Thanks that's perfect! Will call when the time comes.