I know that. You are not understanding my question. I have a bit of Savanna sandwiched between several other biomes (pains, beach, ocean) and I want to change only the Savanna in the selection to plains. This is not about replacing blocks, I am perfectly happy with the actual blocks that are there; I just need to change that Savanna without affecting the surrounding biomes. A selection and //setbiome will change every block inside the selection to the new biome (changing the biome of the block, I am aware it's not the block itself) rather than just Savanna blocks.
TileEntity seems to have understood the problem. If a poly-selection is the only way, then I guess there's no biome mask/filter for //setbiome.