When you have the /fill command (/fill x1 y1 z1 x2 y2 z2 mincraft:...) you can fill things very quick, from at least 1.11.2 untill 1.12pre-7 you could still do the following command to replace a block for a new one, so: /fill x1 y1 z1 x2 y2 z2 mincraft:(the new block)... replace minecraft:(the old block)...
This doesn't work any more in minecraft 1.12