A J to F upgrade is going to be more beneficial on a 777 than 747 IMHO due to escaping the 'flying dorm'. No such worries on a 747.
It's all relative. If the OP has 120 million miles I wouldn't hesitate. If they have 25,000 then I'd save them for a W to J MFU instead.
Originally Posted by
KenJohn
CW let you use the JFK T7 FastTrack queue
If it's open. Which it certainly wasn't last night at the time the LGW/first LHR evening flights/OpenSkies were all checking in