Thank you all very much :)

Happy hunting! :D

Unmodded, you can find the new content on unexplored mountains.

With the Upgrade World mod, you can regenerate explored mountains with the new content.

Back-up your save files before you do anything.

I had the same issue, as Ginseng says that Mod will do the trick AND once its done its thing you can use the refreshed world unmodded if that was the only mod you used. So I have a Thunderstore profile with just that Mod in it now so I can do a world revamp any time they do major content. I downloaded the DB off G-portal ran it modded and experimented till I had the process down, tried it tested it in vanilla mode then put the db file back up to the Server all perfectly.

You can find the new caves on any unexplored mountain biome. So no, you don't need to create a new world, but you DO need to find a mountain biome that you haven't already been to in order for the caves to be there.

Also to clarify things, the "undiscovered" area(ie fogged over on the map) needs to be a good distance from your uncovered area already. If you, for example, uncovered the area around a mountain but not the mountain itself, there is a high chance that no caves will be there even if on a new version of the same map that there normally would be.

The system is more liberal with what it considers rendered previously for a map than the actual fogged out area.

So as long as the mountain biomes are decently far from exposed map areas, and they're meant to have caves(as not all mountains do) then there should be no need to hit a new map.