You can use ur current worlds but the new stuff will only appear in location you haven't been too and haven't been revealed on the map. About 100 m clearence from the borders of what you explored can generate with the new content. Of course, the only things new appearing in the world as far as I'm aware of is onion seeds spawning in chests in mountains and of course the tar pits.

thanks a lot !!! i will star a new one :D