Generate new caves for the cave update in old worlds

I think there should be a option in world settings to generate a new underground systems. This would be available to pre-1.7 worlds, this would be a one time use. This is useful for players with worlds that have unloaded chunks that are too far away so players won't have to create new worlds. You can choose which Y level the new generation begins so as not to destroy any underground builds. I think this would be a nice tool so people won't have to abandon their long term worlds.