J seats are not buy one, get one free. You are welcome to go back and hang out next to your friend in Y if there is an open seat though. Not trying to be snarky, but you understand that it is a problem if everyone in J expects to upgrade a friend for free. Also, I'd also like to upgrade myself to an open seat in J. I'll make friends with my seatmate post hoc.

I'd expect a speedy request that everyone sits in their ticketed cabin other than for a very brief check-in. And yeah. The FAs will notice.

ETA: Wasn't there a Seinfeld episode about this?