Index: src/wp-includes/nav-menu.php
===================================================================
--- src/wp-includes/nav-menu.php	(revision 28727)
+++ src/wp-includes/nav-menu.php	(working copy)
@@ -320,22 +320,44 @@
 		return $menu;
 	}
 
-	$defaults = array(
-		'menu-item-db-id' => $menu_item_db_id,
-		'menu-item-object-id' => 0,
-		'menu-item-object' => '',
-		'menu-item-parent-id' => 0,
-		'menu-item-position' => 0,
-		'menu-item-type' => 'custom',
-		'menu-item-title' => '',
-		'menu-item-url' => '',
-		'menu-item-description' => '',
-		'menu-item-attr-title' => '',
-		'menu-item-target' => '',
-		'menu-item-classes' => '',
-		'menu-item-xfn' => '',
-		'menu-item-status' => '',
-	);
+	if ( 0 !== $menu_item_db_id ) {
+		// Existing item; only changed properties will be updated.
+		$item = get_post( $menu_item_db_id );
+		$item = wp_setup_nav_menu_item( $item );
+		$defaults = array(
+			'menu-item-db-id' => $menu_item_db_id,
+			'menu-item-object-id' => $item->object_id,
+			'menu-item-object' => $item->object,
+			'menu-item-parent-id' => $item->menu_item_parent,
+			'menu-item-position' => $item->menu_order,
+			'menu-item-type' => $item->type,
+			'menu-item-title' => $item->title,
+			'menu-item-url' => $item->url,
+			'menu-item-description' => $item->description,
+			'menu-item-attr-title' => $item->attr_title,
+			'menu-item-target' => $item->target,
+			'menu-item-classes' => $item->classes,
+			'menu-item-xfn' => $item->xfn,
+			'menu-item-status' => $item->post_status,
+		);
+	} else {
+		$defaults = array(
+			'menu-item-db-id' => $menu_item_db_id,
+			'menu-item-object-id' => 0,
+			'menu-item-object' => '',
+			'menu-item-parent-id' => 0,
+			'menu-item-position' => 0,
+			'menu-item-type' => 'custom',
+			'menu-item-title' => '',
+			'menu-item-url' => '',
+			'menu-item-description' => '',
+			'menu-item-attr-title' => '',
+			'menu-item-target' => '',
+			'menu-item-classes' => '',
+			'menu-item-xfn' => '',
+			'menu-item-status' => '',
+		);
+	}
 
 	$args = wp_parse_args( $menu_item_data, $defaults );