It's actually been that way for a little bit. I'd been looking at a coach ticket for earlier this month, and I couldn't get into the J cabin on the coach ticket. Not sure when it changed, but tomorrow's flight is the same way too.

As to who gets the "real" F seats, it's probably first come first serve. If you pick a J cabin fare, it gives you access to both F/J. Since they sell Caribbean as J and not F, it's really a question of who gets the J seat and who gets the lucky upgrade to F.