Change Custom Post Type Name Safely.

Just replace the second line’s post type names:

global $wpdb;
$old_post_types = array('old_type' => 'new_type');
foreach ($old_post_types as $old_type=>$type) {
    $wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->posts} SET post_type = REPLACE(post_type, %s, %s) 
                         WHERE post_type LIKE %s", $old_type, $type, $old_type ) );
    $wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->posts} SET guid = REPLACE(guid, %s, %s) 
                         WHERE guid LIKE %s", "post_type={$old_type}", "post_type={$type}", "%post_type={$type}%" ) );
    $wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->posts} SET guid = REPLACE(guid, %s, %s) 
                         WHERE guid LIKE %s", "/{$old_type}/", "/{$type}/", "%/{$old_type}/%" ) );
}

Source: https://wordpress.stackexchange.com/questions/97026/how-do-i-safely-change-the-name-of-a-custom-post-type