You probably will need a new world, since new stuff can't really be pushed onto explored areas without having maybe adverse effects on the player. Especially when they've already modified it extensively.

That said, there is a mod that allows players to un-explore places they've already... explored.

Not sure if you want to use such a thing, though.

I just created the same seed again. Was ok, since I still had to beat Bonemass at that time. Now, preparing for Yagluth, I would just keep my world and see what comes out of it. I can always create a new world to explore new content with my big character.