The new stock will have lower capacity than the current, right?
So presumably for services where the new stock might be used they can't sell the whole capacity of the Mk3s. Of course the sensible solution would seem to be to release those final spaces, if indeed it turns out the old stock will still be in service, closer to the date. But of course whether their system can deal with that is another question.
At the current time I believe the "extra" berths in the old stock are still be used/sold. However, I have seen some unconfirmed suggestions (wibble, or perhaps not?) that as the new stock/transition period nears they will lock certain berths out of use to replicate the capacity of the new stock.
When it's getting very close to roll-out and in transition (with some Mk5 and some old stock) I think this approach would be needed anyway, as they probably won't know
for definite until a few days/week or so out whether it's old or new stock, and they wouldn't want to sell berths based on old stock capacity and then end up having a new rake working it and telling some excited guests delighted they're on the Mk5s that in fact there isn't actually the room for them. On the flipside, you'd think from a pure business perspective if they've sold based on "new" capacity but it's old stock then it would make sense to use the 'spare' berths and sell them last minute, or as upgrades to seats...?
That said, part of running as per the new format will be for the Sleeper Hosts to get used to the new make-up of each coach (including the composite aspect) so it may not be quite as simple as "we have a spare berth, let's sell it to someone last minute..."
