<?php

defined( 'ABSPATH' ) or exit;

// transfer option
$options = (array) get_option( 'mc4wp_lite', array() );

// merge options, with Pro options taking precedence
$pro_options = (array) get_option( 'mc4wp', array() );
$options     = array_merge( $options, $pro_options );

// update options
update_option( 'mc4wp', $options );

// delete old option
delete_option( 'mc4wp_lite' );