Tesla Inc. has a plan to fend off cheaper competition from China with a $25,000 electric car. But first it has to overhaul a 100-year-old manufacturing process pioneered by Henry Ford.The company is moving to what it calls an “unboxed” approach, which is more like building Legos than a traditional production line. Instead of a large, rectangular car moving along a linear conveyer belt, parts are assembled simultaneously in dedicated areas and then all put together at the end. Tesla says the chan...