There is a mod called Upgrade World, it can add new content and such to old worlds. I used it to add tarpits, mountain caves, and to upgrade the world to the Mistlands patch biome distribution. I can't remember exactly, but when using to change your world to the new patch distribution levels it basically resets everything beyond 5000m from the worlds center. Anyways, seems to have worked well on my world which was made couple a couple days after the game originally launched. Just make sure to read the instructions and make back ups if you decide to go that route.