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.
Quick Tip: The mod for vanilla minecraft called MCedit can be used to copy-paste pieces of landscape. It works with FTB mod blocks.
1. find the FTB world save and plop it into your vanilla minecraft saves folder; NEVER accidentally open it in vanilla or it will get corrupted.
2. open up MCedit and click on "Load World..." and open up the FTB world save.
3. Now you can make crazy-epic landscapes and buil big structures out of mod blocks!