Originally posted by UltraSmurfBrigade:

Update: I went on the Valheim discord - this method will not work. The only way to add the new content is to use mods. The "Upgrade World" on Nexus mods seems to have this capability.

Or... no need to use mods ;)
Even if, for example, you have a save game that has already uncovered the whole map, you can still explore the new cave content... just world hop ;)
The new world will be completely undiscovered and so the caves may be found and explored. Any loot you find may be carried back to your main map.