Yeah I used Upgrade World Mod to refresh my 10 month old world to the new content. As always worked like a charm then uninstalled it.
Some of the new lands are a bit Archipelago but not all.
Post Mistlands : https://valheim-map.world/?seed=DSbDytX5eY&ver=0.212.7&offset=0%2C0&zoom=0.6

Pre Mistlands : https://valheim-map.world/?seed=DSbDytX5eY&ver=0.207.20&offset=0%2C0&zoom=0.6

Feel free to compare the two: some differences but not too bad I felt.

Yes

Yeah, but they are jacked up small and splintered Mistlands.

For best results you need to make a new world. Or, some say a mod can update your world to use most recent world gen.