Microsoft just released Windows 10 Mobile build 10149 to Windows Insiders on the Fast ring, but if you're planning to move on up, you'll want to exercise some patience during the upgrade. Just like in the last build, the lock screen will appear frozen for about 10 minutes after the upgrade while it migrates your data.
SUPER IMPORTANT TO READ THIS (again): The post-upgrade migration status screen is still missing after upgrading to this build which normally shows you the progress of the post-upgrade migration. After upgrading, the Lock screen will appear frozen without the date and time for up to 10 minutes. Please be patient and let it sit – eventually the date and time will appear and it will allow you to unlock your phone. If you get impatient and restart or mess with locking/unlocking your phone, you'll wind up in a funky state – so just relax and let it do its thing.
So there you have it. If you want to avoid causing any unnecessary issues with your device, try to avoid the temptation of messing around with your phone until the date and time appear on the lock screen.