I think what tekky was getting at was use the "Replace" feature found in many text editing programs... It's about as automated as it gets...
You do a "replace" and it gives you two fields to fill in. One that is what you want to replace, and the other is what you want to replace it with...
So in other words, use the replace feature to replace $HTTP_GET_VARS with $GET. Click the "Replace All" button, and it will automatically find all instances of the phrase $HTTP_GET_VARS and replace them with $GET. Many advanced text editors will even do this for any files that are open, so in theory, you open 10 pages needing changes, run the replace feature, and in about 5-10 seconds, all of your $HTTP_GET_VARS becomes $_GET automatically. Not sure how much more automatic it gets (aside from running a PHP script).