BY FRANK OCKENFELS/FX The one thing we can consistently rely on when it comes to American Horror Story is change. Each season the show changes everything from cast members to location to time period. So just how big must next season’s changes be to inspire FX president John Landgraf to call them “unusually large?” We’re guessing pretty big. Landgraf was pretty careful to maintain an air of mystery around next year’s theme. (Something creator Ryan Murphy insists on.) But he did tease the following : I know what it is, but I can’t [say]. I have to let Ryan do his thing on that. But it’s very different. . . One of the things I love so much about that is that it can be radically, radically reinvented in terms of tone, setting, period, characters, cast. I think there’s going to be an unusually large reinvention in between Book 4 and Book 5 relative to, say, between Book 3 [Coven] and 4 [Freak Show]. The only problem being that what we think we know about Season (or are we callin...