About mistlands, we don't really know how that will be handled.

The last update, with the frost caves, is as described - if you've explored (or gone near) an area, it's 'locked in', but if you haven't been there or in the vicinity, it has the new stuff. So, starting a new world isn't really necessary CURRENTLY.

Mistlands might be handled the same way. It might not. We don't know. I think that if at all possible it won't require a restart.

However, even if you do restart, you can keep your character, and take enough with you to make the new start quite easy -- and you can make the new world with the old seed so it will feel nice and familiar - yet different.

You can also explore the new world with the old character, find out if you like it, and go back to the old one. That's the nice thing with easy character portability.