gistfile1.txt

add_action('admin_init','pro_convert_seo_values');
function pro_convert_seo_values()
{
                
    $args = array(
        'post_status' => 'publish',
        'posts_per_page' => -1,
        'post_type' => 'page'
    );
	
    $posts = get_posts($args);
	
    foreach ( $posts as $post ) {
		// Old values
		$oldseo = get_post_meta($post->ID, '_custom_meta2', true);

		$title = $oldseo['fl_meta_title'];
		$desc = $oldseo['fl_description'];
		$keywords = $oldseo['fl_keywords'];
		
		// Update new values with old values
		update_post_meta($post->ID, '_yoast_wpseo_title', $title);
		update_post_meta($post->ID, '_yoast_wpseo_metadesc', $desc);
		update_post_meta($post->ID, '_yoast_wpseo_metakeywords', $keywords);

    }
}