How will phones be updated? Will there be a notification with option to upgrade from 8.1 to 10 or will it just arrive as an "update"?
As always WC will have you covered on that

But it will most likely be the same as when we had to move from 8 to 8.1, we will get a notification that there is an update available and if everything plays out right you might even be asleep when that happens, which will then allow the phone to shut off automatically at 3 am (if you still have the default time selected) and you will wake up needing to enter your pin to enter the phone and witness a brand new WP environment

Would be a great thing to wake up to huh? lol but let's put the speculation aside and mark January 21st in our calenders and be patient... While we're at it we might as well download a countdown app and pin it to the startscreen