From the October dev blog https://steamcommunity.com/games/892970/announcements/detail/3061877729280651185:

It’s also still too early to say what the implementation of the Mistlands will mean for your existing worlds, but we will keep you updated once we know more.

Ah, missed that part, thank you!

With H&H they also added a lot of small things to all existing biomes, so Devs recommended to restart the world with the new update. But it was also playable with your old save - but without all that small changes in explored areas.

So.. when you want the full experience with every major update you should restart your world with every major update.
If you don't care about these minor changes, than you don't need to restart your world.

For myself i will play on different worlds everytime a major update gets released. Maybe i leave out some updates.. it depends how much time i have in the next month/years.