Always be migrating

  A group of unique robots and teddy bears traveling across the country in a wagon train, set in Old West America

No computer system lasts forever. But the data in the system is almost always more important than the code, and is often still relevant even after the system has reached the end of its expected lifespan. So...Don't wait! Do two things. First, when you load data into a website, keep a copy. Then as you build the site, and the data structures are fresh in mind (and you're building data displays) take just a little extra time and build in some data exports. They might come in handy someday.