As Omokoii's answer to this question suggests, you can use MC Edit to delete existing chunks in an old save so that, once 1.8 is out, those chunks will be generated anew, and they should respect the new version's additional landscape and other world-building features, like villages and the new biome. However, Ullallulloo pointed out that random seeds will generate different worlds in 1.8 than they used to, so deleted chunks will not be recreated exactly as they were.
Does that mean that new chunks will not line up seamlessly with old chunks? Just how abrupt will the transition from the old landscape to the new landscape be?