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.

You can use the Upgrade World mod to 'reroll' parts of the map. https://www.nexusmods.com/valheim/mods/1485

New worlds and old worlds; any new content that revamps a biome's terrain will spawn when you *discover* an area.

If youve already explored an area, any future updates that add to the terrain of that biome will not work.

So if you had already explored a section of Plains, you wouldnt get any tar patches when that patch came out.

Now take the Abomination. Its not terrain, just a monster that spawns; so you can have Abominstions spawn in older worlds once you have the new patch.

This means that in older worlds once Mistlands patch comes out, you will see the proper monsters, but none of the new terrain.

This will likrly apply to any existing Mountains youve explored when Caves are released. Any Mountains you explore after the Caves patch should be able to have them.