I don't think it's necessary at all, but I agree that it might be easier in YA.
Middle-grade books have done it all the way along, though. Think of the Amy, Laura and Veronica Ganz books by Marilyn Sachs, the Babysitters Club, and the series that includes Saffy's Angel (can't remember the author or other titles), to name an older, a medium-old, and a current series.
Sometimes this depends on the publisher, if your series is already under contract. One publisher told me I had to stick to the MC in book 1, for fear of scattering the audience who might not want to read about her sister in book 2. If a contract is already in place for a series as a whole, you can't always do what you want -- unless, of course, you're willing to risk preventable rejection on the subsequent books.
Personally, I LIKE series that switch the MC. I like different points of view and perspectives.