One of the main points John Gardner makes in this Philosophy Bites podcast is that written constitutions are immediately less flexible than unwritten versions, and that they become even less flexible as time goes on, until the conversation has shifted away from the document as a means of codifying normative behavior to one of obsessing over the intentions of the authors. I agree. I also find such obsessing unproductive when it comes to defining normative behavior in the 21st century. Though the text might be fixed, the meaning is not. We must reinterpret (rewrite?) the work in the context of our current reality or risk losing it to meaninglessness.
Perhaps an unwritten constitution is the better way to go. Perhaps. Or maybe write it in sand and shake it up every once in a while? The Etch-a-Sketch constitution method?