For that, you could probably just use NBTExplorer or some similar program to change the world type to (I'm guessing) BIOMESOP, although I don't know which file that would be in, or which entry in it you would need to change. This will change which terrain generator is used for new chunks, so any areas you've already explored (and anything within several chunks of those areas) will remain unchanged.
Gonna try this, I'll report back with findings. Thanks everyone for the suggestions. (that AE2 suggestion was genius.