There are tens of millions of Google Voice users, so it would have to be 10 megavolt knife switch, at least. All kidding aside, every single account needs to be touched and a whole lot of other systems need to be altered. It's like a large city performing a major water main or sewer line replacement; first, you build out the new pipes, pumping stations, junctions, sensors, etc, then you connect the new lines, then you shut off the old lines, and abandon them, all while millions of people are using their toilets.