You can use the `posts_where` filter:

    // Add custom filter
    add_filter( 'posts_where', 'wpse_modified' );
    
    // Fetch posts
    $query = new WP_Query( $lastupdated_args );
        '

where you can define the filter callback as:

    function wpse_modified( $where )
    {
       global $wpdb;
       // Run only once:
       remove_filter( current_filter(), __FUNCTION__ );
       // Append custom SQL
       return $where . " AND {$wpdb->posts}.post_modified} != {$wpdb->posts}.post_modified} ';
    }

Though it would be handy to be able to use this kind of *date queries*:

    $args = [
    	'date_query' => [
    		[ 'after' => 'post_modified' ]
    	],
    ];

and

    $args = [
    	'date_query' => [
    		[ 
                'column'    => 'post_modified_gmt', 
                'after'     => 'post_date_gmt', 
                'inclusive' => false 
            ]
    	],
    ];

That's maybe an idea for a core ticket! ;-)