How to blend (fix) broken biome transitions?

The only way is manually I'm afraid, or generate a new world. My usual tactic when there's a terrain generator update is:

1. Get MCEdit
2. Open my old world, and copy the section I want to keep (usually just my main base, not too big)
3. Open Minecraft and generate a new world using the updated terrain generator
4. Find a big area of ocean in the new world (not too hard to find usually)
5. Using MCEdit again, plonk my section of old world into the ocean, or affixed to the edge of a land mass (you can also put it into a biome of the same type, but ocean is easier)
6. Spend a happy afternoon 'tidying up' the edges and making the terrain look believable again.