Actually in the modern age a new product launch requires much less than you think. The lines these days are designed to run several models concurrently. Like platforms and standardized parts share the line. At VW here in Chattanooga they run the Passat, Atlas, and the new Cross Sport on the same line with multiple parts shared across models. There is a picture somewhere on line that shows the plant in Nitra with a new Defender next to a D5 on the same line. Their systems know exactly which serial number is at each pitch and the parts bins have laser readers with green and red lights so if one vehicle is supposed to have a heated steering wheel and the guy tries to grab a non heated wheel the light on the parts bin flashes red. Design and modeling is done with CAD so the process again is way less difficult and in many cases doesn't require a plant shut down or at least not a complete shut down.