UserUpdate8002.php

<?php

namespace Drupal\user\Plugin\migrate\process;

use Drupal\migrate\Attribute\MigrateProcess;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * Keep the predefined roles for rid 1 and 2.
 */
class UserUpdate8002 extends ProcessPluginBase {
  
  /**
   * {@inheritdoc}
   *
   * Keep the predefined roles for rid 1 and 2.
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $rid = $row->getSourceProperty('rid');
    $map = [
      1 => 'anonymous',
      2 => 'authenticated',
    ];
    return $map[$rid] ?? $value;
  }

}