There's a mod, where you can regenerate your world while keeping some manually set areas.

If you're not afraid of using that mod, you can keep your progress while getting all changes even in already discovered terrain.

Uprade World @ Nexus mods
--> https://www.nexusmods.com/valheim/mods/1485

edit: if the whole world generator gets changed, e.g. for new bioms, old terrain may not be fitting into the regenerated world though.