There are mods you can try to upgrade your World.
https://www.nexusmods.com/valheim/mods/1485?tab=posts&BH=0

I read it should spawn in your old worlds if you don't have most places explored already gotta be under blocked map.

Expansion? This isn't an MMO, there is no "expansion" or "base game". It's all one thing that's still in development and Mistlands is just the next partof that development.

I have used my old world as a test.

Here is how it works:

For old worlds, the boime is more compact, in new worlds it apparently will be wider.

In old the old worlds: Loaded/rendered biomes are a bit larger than what the map reveals.

Previously loaded mistlands will get terrain updates, new hills and lower grounds, monsters spawn but there is no fog, no plants, dungeons or buildings.

Unloaded biomes, will generate all new features as expected.

Just start playing and don't get over involved with a fancy base. Then when Mistlands hits stable just create a new map and transfer your character and any inventory items you want to the new save.

Not sure. There is a new boss stone so I do not know if you will have that on an old world. Any unexplored mistlands in an old world will turn into new mistlands so I don't think you have to restart. Also if you do have to make a new world, you can use it only for mistlands.

When Mistland will be out?