Originally Posted by superscot

Based on my experiences over the past couple of weeks, I would be very careful with this, especially if you have one of the better J seats reserved.
Twice I have seen this on a FLUB 787-9 and moved my seat assignment from 7A to one of the F seats showing as available. Both times, at T-24 my seat has been changed back to a J seat but, as both 7A and 7K had been taken by somebody else by then, I was left with a much less desirable J seat.
When I queried this at check-in at DEN, I was told that if loads were sufficiently light so that the F cabin wasn't needed, at t-24 they would block the F cabin for crew use and just use the J cabin.