Terraformers should change biomes in chunks, instead of only simulating the biome's effects. Or add some "Permament" Terraformer, that changes the biome until overwritten with a different setting. Also, it would be extremely power-hungry.
Note: Since 1.2, Chunks store biome data inside, making changing biomes much easier. For example, WorldEdit can change biomes in selection by //setbiome biomename.