When No Man's Sky did its first procedural terrain change, the new did replace the old.

Given that the terrain is generated from the algorithm first then player edits are applied to it, if they changed how world generation works, you wont have the old stuff as i don't believe Valheim saves the actual world state at all, just your changes.

Not that i think they will do this, they have placeholders already there for all the future planned biomes so there's no point in upsetting things.