[Greasemonkey] replacing a script with my own

[jjazz]

I frequent a website that loads a script in the head section with 

<script type="text/javascript" src="review.js"></script>

I would like Greasemonkey to get in there and replace review.js with another
url-- i.e. one that points to my customized version.  I am having trouble
getting in there and making the change.  I am doing the following, which
does changes the src of the script tag, but doesn't affect the operation of
the site.

var allScripts = document.evaluate( '//script[@src]', document, null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
for (var i = 0; i < allScripts.snapshotLength; i++) {
    var thisScript = allScripts.snapshotItem(i);
    if (thisScript.src == 'http://url.of.site/review.js') {
      thisScript.src = 'http://url.to.my.version/newscript.js';
    }
}

I feel like I've missed somethig simple (I have little javascript or DOM
experience).  Any hints would be most appreciated.

Thanks.
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