Place a PMPro member in another level when they cancel.

pmpro_cancelled_level.php

<?php
/*
Plugin Name: Eighty/20 Results: Assign free level when cancelling
Plugin URI: https://eighty20results.com/paid-memberships-pro/do-it-for-me/
Description: Custom code to configure a default member level for a user who cancels
Version: 1.0
Requires at least: 4.5
Tested up to: 4.9.8
Author: Eighty / 20 Results by Wicked Strong Chicks, LLC <thomas@eighty20results.com>
Author URI: https://eighty20results.com/thomas-sjolshagen/
License URI: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
License: GPLv2 or later
*/

/**
 * Set the membership level ID your member should be assigned when they cancel
 */
if (! defined('PMPRO_DOWNGRADE_TARGET' ) ) {
    define( 'PMPRO_DOWNGRADE_TARGET', 1 ); // TODO: Change to the correct membership level ID
}

/**
 * Change membership level when the user's membership is cancelled/expired
 * 
 * @param int $level_id
 * @param int $user_id
 */
function pmpro_after_change_membership_level_default_level($level_id, $user_id) {

	// If we see this global set, then another gist is planning to give the user their level back
	global $pmpro_next_payment_timestamp;

	if ( ! empty( $pmpro_next_payment_timestamp ) ) {
		return;
	}
	
	if ( 0 == $level_id ) {
		// Actually cancelling, give them PMPRO_DOWNGRADE_TARGET membership level instead
		pmpro_changeMembershipLevel( PMPRO_DOWNGRADE_TARGET, $user_id);
	}
}

add_action("pmpro_after_change_membership_level", "pmpro_after_change_membership_level_default_level", 999, 2);