Well it's only one experience, but I'm sitting here in The CX lounge at LHR still on a wait list for an upgrade on Flt. 105. EF is still showing J2 C0. They confirmed that there were still 2 J seats for sale when I checked-in. First time this has ever happened to me on this flight (which I fly fairly often).

In my experience, even on very full flights on this route, they will release any unsold J the night before. Hope it's not a trend to hold all unsold J seats right up until departure, or even (shudder) not releasing them to C at all!