Manu item types in Mambo/Joomla cms are unique in the sense that it determines how to display the contents under a particular menu item. Though there are many different types of menu items, but, i’ve used only three of them.
1. Link URL : When you create a menu, if you select Link URL as the menu item type, an href link to the target url is created, and when someone clicks on that menu, the target url opens up.
2. Content Table: Selecting this menu item type will display all the content items under the menu in a tabular form, like, Date created, Title, Hits, and so on, by default.
3. Blog Section: This type of menu item shows the content items under them in blog format, with intro text and a live link to the detailed part, by default.
But for any reason whatsoever if you want to change the menu item type, so far as my knowledge goes, it is not possible to do so. The only option is to delete the existing menu item and create a new one with the same name with appropriate item type selected.