Upgrade World, that simple.
Sure you could lose something you've changed, but the new stuff gets added in, so it's worth it.

New stuff does show up in old worlds. My world was created the day the game released. Onions are found in chests.

Mountain biome areas that were explored and uncovered prior to the inclusion of Frost Caves will not contain Frost Caves. Mistland areas explored and uncovered prior to the addition of Mistlands update generally will not contain Dvergr structures or Infested Mines, but usually contain Mistland creatures.

If you have a decent amount of the world unexplored, new content will be included in those areas.

https://www.nexusmods.com/valheim/mods/1485?tab=description

Start over with one of the discovered seeds that are very useful...if you don't feel like leveling up at the start, you could choose to devcommand yourself "to the point where you were at", sort of...but yeah, I would just start over with a new world

you can definitely mod your stuff back in.

the rule is that what you have seen, including some small buffer past that, will not update.
so find a new mountain you have not visited and have at it. Im on an ancient world and I have all the new content through mistlands.

well buckle my boots i finally found a cult cave on a mountain.

I would make a new world if this is an old one

Download the Upgrade World mod and you can reload existing worlds with a console command, just added in coral reefs from fishing mod into my old world and it worked perfect.

New stuff won't show up on an old save. You could make a new one with the same seed, and it will recalculate it with the new biomes. Not sure how similar it will end up