Two versions of jQuery in a restrictive situation.

header.html

<!-- At this point our environmental jQuery should have already loaded, we'll
assign it to a separate variable on the window and delete the dollar sign -->
<script>
  window.jQuery142 = window.jQuery;
  delete window.$;
</script>

<!-- Next, we'll load our next version of jQuery -->
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>

<!-- Finally, we'll set our newer jQuery into noConflict mode and assign it to
an alernative variable, assigning our environmental jQuery back to window.jquery
and dollar sign -->
<script>
  window.jQuery1102 = window.jQuery.noConflict();

  window.jQuery = window.jQuery142;
  window.$ = window.jQuery142;
</script>

<!-- Now I can add scripts to the site which are written for modern jQuery -->
<script>
jQuery(document).ready(function($) {
  // our existing environmental jQuery will use the old version
  // If we use a normal ready statement we'll also use the old version
});

window.jQuery1102(document).ready(function($) {
  // If any of our code references window.jquery we'll cover ourselves by setting
  // it to the new jQuery for a bit
  window.jQuery = window.jQuery1102;
  
  // All my modern jQuery code goes here
  
  // And we'll reset the window jQuery now that we're done
  window.jQuery = window.jQuery142;
});
</script>