You can certainly ask the crew to swap seats if you want to but I didn't even bother to when I did a similar thing. Brother and I flew SIN-BKK. I booked him in Y, myself in J. Swapped boarding passes on the airbridge just before entering aircraft, no need to speak to cabin crew. He enjoyed the flight up front. Also, as we share the same surname no problem with crew addressing him by name.
So you could easily do this once you get onto the airbridge at JFK and swap back in FRA. Or just ask the crew.
Originally Posted by
NYBanker
Plus, if you ask nicely, you should be able to bring the J pax to F for meals (and sit in the guest pod).
Don't even think about doing that. First Class pax can visit the J cabin, not vice versa. The crew will make sure of that.