DJ use complimentary upgrade to J, @ T-6?

[sail0r]

Just booked a heavily sold flight in Y. Called up to try to use WP upgrade to J (only 1 J seat left showing in flightstats).

Told not available, when asked why they were still available for purchase, told they keep 1 for sale, understandable.

I asked if they release it at, say, T-6, they said there is no set time, but must process upgrade before T-4.
Asked if they had a waitlist,
no

Q: does anyone know if it is a set time, say, T-6, when they might release this last J seat?:?:

 

[mella2007]

I've had cases where they never release the seat and it sits there for the entire flight.

You can only call or see via Check in or The Lounge if can they assist you better at all.