I always book the bulkhead seats on long flights in Y, it allows me to get up easily from the window seat. I've had 2 different experiences over the years aside from the normal one (flew in the bulkhead seat)

The first was overhearing a couple arguing with a gate agent about basinet seats, followed by being asked on-board by the couple to switch with them in Y-. Both I and the passenger next to me made the switch and we were allowed to use an empty pilot rest seat, I don't recall if it was in J or F for this flight, for about 4 hours when one of the seats was free. I would not count on that happening again.

The second was losing my bulkhead seat, not sure when it happened, but I ended up with a departure management card at the airport. My fault for not checking on my reservation. Worked out fine as I was given one of the few window seats with an empty middle, but there was also the possibility of being bumped.

Not sure what the official policy is, but I would be prepared for a small chance of either of the outcomes I experienced if you book a bulkhead.