Adding new world Generation Mod to Minecraft?

If you add the mod, and keep the world, any chunk generated from that point should generate with flat bedrock, but all the chunks that have been generated probably won't change. I would just try it without deleting the world and see what happens, if you aren't happy with the results, you could delete the world folder, or just rename it and save it for later.

You came change a server's landscape without starting a new server. :/ sorry buddy