https://twitter.com/#!/jeb_/status/154928078777356289
Somewhat good news. According to Jeb, differences between the generator should be minor enough that you shouldn't encounter too many differences between old chunks and new.
Hopefully. If you want to make sure the world looks as appealing as possible though, you all might want to go out and explore and generate as many chunks as possible.
On the other hand, if anyone has a 1.0 single-player world that's fairly well explored, you can try out the new snapshot and explore new areas and let us all know how the transition fairs.
However, even though he said he only changed the variation, he also changed the range of heights in biomes. I feel that you might see a lot of "walls" of blocks on newly generated chunks because of the differences in height.