Pretty sure any undiscovered mistland biomes you have on your world will automatically update. So unless you discovered and explored every biome or mistlands biome in this case, you should have a few that update. But that i just heard that around the forums.

Hey there,

you can find that answer in the FAQ post:
https://steamcommunity.com/app/892970/discussions/0/3383905288030494726/

Have a good day!