That's slightly hyperbolic. I don't think they intend for the primary use case to be for people to upgrade in place, like updating a nightly Firefox, but rather to start new projects with the new version. But previous big upgrades have been largely a bunch of search and replaces. There's various tools that people have written to automate this. Again, I wouldn't blindly run it and expect everything to work, but it's far, far less than a complete rewrite.
We all do tend to do a redesign of our sites and apps eventually and that the correct time to make the transition.