Well, as long as all threads started before the deadline still expire (because people will have posted on them in expectation that they will)
And they need to make it bloody clear during the transition (different colour, perhaps?) whether the thread is going to be ephemeral or permanent.
I think they've got this wrong though.
Ephermeral chat is a good thing. And I'd make AIBU ephemeral too. There are plenty of topics for those who are looking for worthwhile advice.