String Replace Caching & Chance of Collisions

You would probably get at least equally good performance by simply adding the classes with a filter, and it is certainly neater and closer to the way the WordPress is meant to be modified:

function add_classes_wpse_190966($classes) {
  if (in_array('current-menu-parent',$classes)) {
    $classes[] = 'active';
  }
  return $classes;
}
add_filter('nav_menu_css_class','add_classes_wpse_190966');

Or, with an array of values:

function add_classes_wpse_190966($classes) {
  $itemsToReplace = array(
    'current-menu-parent' => 'current-menu-parent active',
    'item' => 'name'
  );
  $classes = array_intersect_key(array_combine($classes,$classes),$itemsToReplace);

  foreach ($classes as $v) {
    $classes[] = $itemsToReplace[$v];
  }

  return $classes;
}
add_filter('nav_menu_css_class','add_classes_wpse_190966');