menu_set_item function not altering menu item of specified path

API page: http://api.drupal.org/api/drupal/includes%21menu.inc/function/menu_set_item/7

Enter a descriptive title (above) relating to menu_set_item, then describe the problem you have found:

This is my first potential issue. I will do my best to follow protocol. I may be misunderstanding the behavior of this function. If so, I apologize.

Repeatable?

I was able to repeat this in multiple page callback functions for different menu items.

What are the steps required to reproduce the bug?

I have a link in hook_menu which can change based on the page accessed.

      $items['intro-courses/course/section/course-id'] = array(
		'title' => 'Add Section',
		'page callback' => 'drupal_get_form',
		'page arguments' => array('intro_section_form'),
		'access callback' => 'intro_phys_administrator_access',
		'menu_name' => 'main-menu',
		'type' => MENU_NORMAL_ITEM,
	);

From the above code, the course-id in the path can change based on the rendered page. In the page callback of the rendered page, I call menu_set_item with the following code...

//get the cached menu item
$item = menu_get_item('intro-courses/course/section/course-id');
//debugging purposes only
dpm($item);
//retrieve its path (should be the same but sanity check)
$old_path = $item['path'];

//alter the link
$item['href'] = 'intro-courses/course/section/add/'.arg(2);
//change the path
$item['path'] = 'intro-courses/course/section/add/'.arg(2);
//update the map
$item['map'][3] = arg(2);
//doubtful i need to update the original map, but doing this for testing
$item['original_map'][3] = arg(2);
//updating the title of the link
$item['title'] = 'Please change';

//attempting to update the menu item
menu_set_item($old_path, $item);

/**

//the following line seems to work as expected when no path is passed
menu_set_item(NULL, $item); 

**/

What behavior were you expecting?

I was expecting that the page callback function would update the altered attributes menu item specified by the path parameter before page load. It did not seem to change anything. I was able to successfully overwrite the object when the path parameter was set to NULL.

I have the children menu items in main-menu rendering in the theme. The menu item I am trying to alter is a child item of the rendered page, but changes based on the rendered page.

What happened instead?

The rendered page left the specified menu item unchanged.