Quote fromQuote fromI had an idea that could work (and could make an enormous mess!):
Scan through a map and make a big list of all the chunks that are defined.
Re-generate all those chunks using the same random seed with the pre-halloween minecraft to create a clean copy
Go though chunk by chunk comparing each block of the map with the "clean copy" version.
For any blocks that are the same, set them to some invalid block ID (like 100)
Now, using the current version, generate a map with the same seed and the new generator.
Copy all blocks with a valid id from the old map to the new map.
This will fill in all your structures and even caves you've chipped away, superimposed over a fully regenerated map, complete with biomes! It might work almost perfectly in some places, it might make a HUGE mess in others. But I think it would be a pretty neat way to convert an old level to a new one with full biomes (and making some pretty wild artifacts in the process).
I am not entirely sure what you talking here, but in theory it is possible to generate a pre-halloween terrain with the new update and fully functional biomes. You would just need to copy the map seed from the old pre-halloween level.dat (The whole file? One line in the file? I do not know. But I fixed my world's changed map seed by replacing the level.dat file so the seed is in fact somewhere in there.) and replacing a new post-halloween worlds map seed. Then simply deleting all the chunks with a map editor. I gotta try if that works today.
I'm talking about doing that but without losing anything you changed about the old terrain (buildings, tunnels, etc). Basically generate a clean copy of your current map with the old generator, taking the difference, then pasting the changed chunks over a newly generated world with biomes. Though it appears the terrain generator that supports biomes creates different terrain for the same seed, so it might make a mess.