I think it will be a rolling release of records. At least, I don't think it should be an issue for them to do it.
The FAQ says:
"we have developed new methods of automatically closing the records of those people who are living and younger than 100 years and one day."
I would imagine if it's automatically closing the records, it will be able to automatically open them.
Why haven't they put this on the 1939 site, I wonder?
The email they sent me seems to almost be a copy and paste job - it sort of repeats itself here and there and has an odd flow to it - just my opinion

It doesn't feel like a prepared email for these requests, so I think they're still a bit behind the eight ball in getting things set up.
I don't know if anyone else has noticed that "Findmypast Friday" quietly didn't happen last week (though the electoral registers are more than enough to tide me over), the blogs have been revamped so that you can't actually view them chronologically anymore as far as I can see, and there's a new "latest news" box that has had the same news about the electoral registers being uploaded 1 minute ago for a week or so.
I reckon it's all hands on deck trying to get things all together.
On October the 1st, "Findmypast First" advertised a webcast slated for 3pm GMT on October 30. It promised:
"We'll be taking you on a journey through an exciting new record set released later this month."
Unless they meant the electoral records, I'm guessing the register was due for the weekend (which was still "this month") but ran behind. Monday is an odd day to release something that many people won't be able to access until after work.
This should be an interesting watch, though it's 4am Australia time, so I hope they upload the webcast somewhere afterwards.