<?php
defined('ABSPATH') or exit;
// transfer option
$options = (array) get_option('mc4wp_lite', []);
// merge options, with Pro options taking precedence
$pro_options = (array) get_option('mc4wp', []);
$options = array_merge($options, $pro_options);
// update options
update_option('mc4wp', $options);
// delete old option
delete_option('mc4wp_lite');