[quote=Giygas]
I may be wrong, but I think rabbits will still spawn if we use this world. Horses are different and spawn as the chunks generate. Rabbits use the same spawning mechanics as pigs, sheep, etc. Which means they spawn on grass randomly. And the only other thing is underwater temples, which will not be able to spawn. (I've heard updated generation occurs 1,000+ blocks from where anyone has generated chunks, but I'm not sure). Maybe we should give the world a chance before resetting everything.
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As a player of modded MC(Trust me, tons of experience with new world gen), new world gen will occur immediately after new chunks are generated. However, there may be some visible edges between new and old chunk generation(For example, you might get a giant flat wall of stone leading on to an plains biome). But other then that, it should be safe.