Alghaster

Hi folks, I'm looking to add a mod to my daughter's server that will make combat a bit more interesting. I've looked over a number of them and have decided on one called Power of the Elements ( http://www.minecraftdl.com/power-of-elements-mod/ ). I really like the look of it, however, I have a question. This mod adds new block types to your world, and we are playing on an existing world. Can you install a mod like this on a pre-existing world? It doesn't seem like you could to me, since the world has already been rendered and the mod adds new blocks, but I wanted to verify. If it isn't possible, can anyone suggest fun combat mods that you can add to an existing server? Thanks