Show a 'Notify me' button, and hide the 'Add' button.

gistfile1.html

<!-- Updating the product.liquid HTML to have a hidden Notify Me button -->

<!-- START BUY -->
<div id="buy" class="clear">
  <h2 id="price"><span>$135.00</span></h2>
  <input class="button" type="submit" id="add" value="Buy Now" />
  <a href="#" id="notifyme">Notify when available</a>
</div>
<!-- END BUY -->

gistfile2.css

/* Then hide the notify me button by default */

#notifyme { 
  display: none;
}

gistfile3.js

// Update the selectCallback to show or hide the Add button and Notify Me as appropriate.

var selectCallback = function(variant, selector) {
  if (variant && variant.available == true) {
    $("input#add").show();
    $('#notifyme').hide();
    $("h2#price span").html(Shopify.formatMoney(variant.price, "${{amount}}"));
    if (variant.compare_at_price) {
      $("h2#price del").html(Shopify.formatMoney(variant.compare_at_price, "${{amount}}"));
    } else {
      $("h2#price del").empty();
    }
  } else {
    $("input#add").hide();
    $('#notifyme').show();
    var message = variant ? "Sold Out" : "Unavailable";
    $("h2#price span").text(message);
    $("h2#price del").empty();
  }
};