So.... It didn't work. xD The world didn't regenerate according to the original seed at all, and it didn't solve the abrupt-transition-vertical-wall problem... It just created those vertical walls in a box around the area I kept in the pruning process, as shown in the image. What could be going wrong?
Hmm... Could be a problem with the seed?
Try using an NBT editor of some sort (like NBTExplorer) and check the level.dat to see if the seed is correct. If it's not, try changing it to the correct seed.