just a weird idea: would it be possible to adjust sealevel? once it was set to 64 because the world height was 128. but now the world is 256 high, and sealevel is still at 64. I love to dig around, the deeper the better...
if the sealevel is at 100 or 128, it still leaves a lot of space for extreme hill biomes.
of course, a setting for that needs to be done at the first time, a game is started, else it would leave the old chunks at sealevel 64, and the new ones are suddenly not compatible to the old ones.