Originally posted by Eekhoorn:

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.

Who knows, they could introduce new resources, recipes, work stations as the game gets developed. Even if the focus is on Mistlands I could see other large changes on each major patch which could cause all sorts of problems, but who knows. We will know on each patch.

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.

only if that biome/part of the world its unexplored ( still with fog in the map ) , if you already explore some part of that biome , only THAT PART will get not update

In H&H, tar pits would not spawn in plains that was already discovered, but would in plains yet to be explored.
I heard, cant remember where, that the current plan for mistlands is not to change any mistland biome already discovered...

You should be able to keep an old seed and still have the new contents, but only in areas not yet explored, probably even be able to use the same seed to re-create the world (but you would need to explore and re-build again, but if you enjoyed that seed it will give you a heads up).