The change to 744 takes effect on 2 May 2017.
Proper F class inventory is loaded on the flights, and TG has filed an F fare for the route, so I'm not sure why it doesn't appear when you search.
Either way, it looks like they intend to operate F when it switches over to the 744 (unless they change their mind last-minute, which would be far from surprising). I would book O class without worrying about whether you will get to fly F.