How to force a script reload and re-execute?

How about adding a new script tag to <head> with the script to (re)load? Something like below:

<script>
   function load_js()
   {
      var head= document.getElementsByTagName('head')[0];
      var script= document.createElement('script');
      script.src= 'source_file.js';
      head.appendChild(script);
   }
   load_js();
</script>

The main point is inserting a new script tag -- you can remove the old one without consequence. You may need to add a timestamp to the query string if you have caching issues.

Here's a method which is similar to Kelly's but will remove any pre-existing script with the same source, and uses jQuery.

<script>
    function reload_js(src) {
        $('script[src="' + src + '"]').remove();
        $('<script>').attr('src', src).appendTo('head');
    }
    reload_js('source_file.js');
</script>

Note that the 'type' attribute is no longer needed for scripts as of HTML5. (http://www.w3.org/html/wg/drafts/html/master/scripting-1.html#the-script-element)

Have you tried removing it from the DOM, then inserting it back again?

I just did, that doesn't work. However, creating a new script tag and copying the contents of the existing script tag, then adding it, works well.

See my example http://jsfiddle.net/mendesjuan/LPFYB/

var scriptTag = document.createElement('script');
scriptTag.innerText = "document.body.innerHTML += 'Here again ---<BR>';";
var head = document.getElementsByTagName('head')[0];
head.appendChild(scriptTag);

setInterval(function() {
    head.removeChild(scriptTag);
    var newScriptTag = document.createElement('script');
    newScriptTag.innerText = scriptTag.innerText;
    head.appendChild(newScriptTag);
    scriptTag = newScriptTag;    
}, 1000);

This won't work if you expect the script to change every time, which I believe is your case. You should follow Kelly's suggestion, just remove the old script tag (just to keep the DOM slim, it won't affect the outcome) and reinsert a new script tag with the same src, plus a cachebuster.

Small tweak to Luke's answer,

 function reloadJs(src) {
    src = $('script[src$="' + src + '"]').attr("src");
    $('script[src$="' + src + '"]').remove();
    $('<script/>').attr('src', src).appendTo('head');
}

and call it like,