You can keep your old world. Mistlands will still be there, in completely unexplored areas.

But...the zones will be smaller and may be harder to navigate. It's a matter of luck how well they work out. If you want to take the chance, you will have better luck finding a large area to the east or west, since large chunks of north and south are taken. The map generation will change with the update.

If you don't have a huge base, then I recommend moving your character and your stuff over to a new map. If you are attached to your base, then you can take a chance on the old map, or you can send a "raiding party" to a fresh map.

Added: If you want to reuse the same seed on a fresh map, the land masses will be the same, but the biomes will shift a little and the bosses will move from what I've seen from looking at the updated map website.

This is not exactly true. Yes you can keep your old world. Yes Mistlands will generate in unexplored areas. HOWEVER. Those Mistlands will be much smaller and filled with way more jagged rocks and small islands do to a new system they are using to generate the world. If you want the new "True" Mistalnds experience, you should start a new world/seed once the update comes out.

https://steamcommunity.com/app/892970/discussions/5/3589961352692584029/

I have the same question, I feel that the link above does not answer my understanding of the question.

To be clear, I think we want to know, come Myst release. Will we be able to keep our old worlds and evolve them into Myst or will we need to create a new world when the next expansion releases ?

The only thing that kinda comes close to answering the assumed question is as follows:

"Is it required to start a new save on the Public Test branch?
No! You can simply continue with your normal Early Access save. However, please note that new content does not appear in already explored/generated areas. There are some important other things to take note of before switching versions as well (read the three questions below)."

Originally posted by Pixel Walker:

I have the same question, I feel that the link above does not answer my understanding of the question.

To be clear, I think we want to know, come Myst release. Will we be able to keep our old worlds and evolve them into Myst or will we need to create a new world when the next expansion releases ?

The only thing that kinda comes close to answering the assumed question is as follows:

"Is it required to start a new save on the Public Test branch?
No! You can simply continue with your normal Early Access save. However, please note that new content does not appear in already explored/generated areas. There are some important other things to take note of before switching versions as well (read the three questions below)."

U answered ur question in ur post.

How does that not make any sense.

No u do not need to start a new world. Unexplored mistland zones will spawn in the new content. Explored mistlands won't

Originally posted by Pixel Walker:

I have the same question, I feel that the link above does not answer my understanding of the question.

To be clear, I think we want to know, come Myst release. Will we be able to keep our old worlds and evolve them into Myst or will we need to create a new world when the next expansion releases ?

I don't remember where I saw it, but the devs mentioned that you shouldn't have to start a new world. The world in Valheim exists in a strange sense, where only areas within 200 meters of you get created and set in stone. Meaning, if in your current world you have been within 200 meters of a Mistlands biome, it will have been created in its unfinished state and will never change from that. Any Mistlands biome you haven't been within 200 meters of, on the other hand, has not been fully generated yet and will be able to be explored once Mistland drops to the default branch.

You can probably check this by making a backup of your save, using the console map to reveal the map (I'm pretty sure one exists), and see if you have any Mistlands biome close to where you currently are. Do be sure to make backups though, and to replace the file with your backup once done. I've heard that using that method can set the biomes to count as explored, meaning they'll never populate with yet-to-be-released content.

Note: I'm not 100% sure it's a 200 meter range, but something like that.