Editing a Drupal 7 sub-themes menu HTML
You just want to make the links show as uppsercase text!? Since you're not altering the HTML structure of the menu -- there is no need to modify any theme function. You just do it using CSS.

/* In ideally your last css file listed in your .info file
 * add a line such as
 */
#main-menu a { text-transformation: uppercase !important; }