UserUpdate8002.php

<?php

namespace Drupal\user\Plugin\migrate\process;

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

/**
 * Keep the predefined roles for rid 1 and 2.
 *
 * @MigrateProcessPlugin(
 *   id = "user_update_8002"
 * )
 */
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;
    }

}