Selected answers from the Dumb SEO Questions G+ community.
Jacob Punton: Put it into dev first, then put it on the live site with all the 301s in place, then set the home and site URL to the new one via the database upload. That way it all goes live at the same time.
Jim Munro: Is a change of url structure really necessary? What is the hoped-for outcome?
Tatsat Savsani: Have the a code prepared with a list of url changes and do the change.. Once done add it to htaccess.. It does need to be that instant but sooner the better