Newest items are first.
This is different from the login stuff, where the newest is last. If you use pine or mutt to check your mail, or don't use mail at all, this won't affect you. If you use POP to check your mail from an email client or a webmail service, this also won't affect you. That shouldn't be very many of you. I plan to do this around 8-9pm Pacific time, and it shouldn't take more than half an hour or so to see for certain if the changes are staying in place.