You indeed can change the Menu Item's Type to whatever you want, even after they have been created.
If, for instance, you want to change the Blog Section of a Menu link, go to the Control Panel->Menus Menu->[menuname]->Menu Item Manager and edit the Menu Item. Select the Change Type button and choose the new style of Menu Item Type from the available list. Thereafter, alter the Details and Parameters to reconfigure the display for the new selection as you require it.
To completely remove an Article, select the Articles that you want to delete and move them to the Trash. Next, open the Article Trash in the Content Menu and select the Articles you want to delete. After deleting an Article, it is no longer available as it has been deleted from the database and it is not possible to undo this operation.