If you're a Tesla buyer, you were probably miffed when you realized that the self-driving hardware revision rendered your car obsolete, even if it was just weeks old. However, you're going to have to get used to that feeling if you're going to stick with the brand. Tesla founder Elon Musk is telling customers that there will be "major" revisions every 12 to 18 months -- not quite yearly, but far more frequently than the every few years (at best) you see from conventional companies. He adds that retrofits would slow the company's progress "dramatically." For example, adding self-driving tech to earlier vehicles would require stripping the "entire car" and replacing 300 parts.