HOWTO: Regenerate an old world with the new biomes (keep your items, not your buildings)

This is a fun way to recreate your familiar world in the boo update without deleting chunks and making your world look messy.

Important: Your new world will lose all your man-made buildings and mines if you follow this! This is the reset button for your main world. Treat it like the apocalypse and you're returning to your world eons in the past or future to forge a new home.

See my landmark screenshots here to see differences: http://sirthorax.imgur.com/biomes
Testimonial from MassesOfTheOpiate who tried something like it: http://www.reddit.com/r/Minecraft/comments/e0c3v/biomes_in_action_beforeafter_pics_of_an_old_world/c14b8eo

1. Create a portal (ideally it's a very accurate portal at a height to get you back to your spawn location in the real world). Recommended: create lots of different portals in the Nether for rapid exploration.
2. Gather a compass and every belonging you want to "time travel" with you-- take it and place it into safe chests in the Nether world (ideally near the portal you want to use).
3. Be sure you're standing in the Nether before you start your trip
4. Save/Exit Minecraft
5. I recommend you copy your .minecraft/saves/WorldX directory into a new World directory (e.g. if World1 copy it into an empty slot, perhaps World5). If you try to do this operation in-place on the same world, be sure to backup your World directory in case you hate this and want to go back.
6. With the new World copy, go inside and delete all the chunk subdirectories being careful not to delete: (a) the DIM-1 subfolder (your Nether) and (b) any .dat files-- especially your level.dat file(s)
7. Load Minecraft and load that new World savegame, you should be in the Nether with everything still there as you left it.
8. Step through the nearest portal and search for all the eerie remnant landmarks of your old world.
9. ???
10. Profit!

If you want to import buildings, some Minecraft editors allow importing schematics, etc. You can also try to carefully import specific chunks from other save games, but I've not tried that myself (seems tricky and bizarre when elevations don't line up).

TLDR: Backup savegames. Go to Nether with your stashes, duplicate your savegame, delete all files but DIR-1 and level.dat files, reload minecraft, exit portal. Voila, new "familiar" biome world without all your buildings.