transforming uid=1 to uid=3, leave the rest the same

migration.yml

source:
  plugin: d6_node
  node_type: ao
process:
  title: title
  uid:
    plugin: transform_uid
    source: node_uid

TransformUid.php

<?php

namespace Drupal\foo_migrate\Plugin\migrate\process;

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

/**
 * Replace uid=1 with uid=3
 *
 * @MigrateProcessPlugin(
 *  id = "transform_uid"
 * )
 */
class TransformUid extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    return ($value == 1) ? 3 : $value;
  }

}