Update (23 Jan 2016, 3:15pm EST): The maintenance is complete! Thank you for your patience.
Usually, I try to schedule downtime in the evening, Eastern time, but because this is a big and delicate change, I'm planning on starting it at 11am, Eastern (16:00 GMT). Currently the window is until 2pm, Eastern (19:00 GMT), and I will update this post if that changes.
I apologize for the inconvenience. It's critically important to be able to recover from the types of issues we saw last week, quickly and without losing your data.