When users expire (and the system changes them to membership level 0) we give them another downgraded level.

my_pmpro_membership_post_membership_expiry.php

<?php
/**
 * When users expire (and the system changes them to membership level 0) we give them another downgraded level.
 * This gist is setup to specifically downgrade level 6 to level 3 with an expiration set to 305 days out and
 * downgrade level 5 to level 2 with an expiration set to 150 days out.
 * Note that this code is using the pmpro_membership_post_membership_expiry hook which only fires when a user is expired
 * by the PMPro cron job. If an admin changes the user's level or they cancel, they will not receive the downgraded level.
 * Adjust the gist to meet your needs.
 * Add this code to a custom plugin. https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_membership_post_membership_expiry( $user_id, $level_id ) {

	// Set this to the id of the level you want to give members when they cancel.
	$downgrade_level_2 = array(
		'user_id' => $user_id,
		'membership_id' => 2,
		'code_id' => '',
		'initial_payment' => '',
		'billing_amount' => '',
		'cycle_number' => '',
		'cycle_period' => '',
		'billing_limit' => '',
		'trial_amount' => '',
		'trial_limit' => '',
		'startdate' => current_time('mysql'),
		'enddate' => date("Y-m-d", strtotime( '+150 Days', current_time('timestamp') ) ),
	);
	
	$downgrade_level_3 = array(
		'user_id' => $user_id,
		'membership_id' => 3,
		'code_id' => '',
		'initial_payment' => '',
		'billing_amount' => '',
		'cycle_number' => '',
		'cycle_period' => '',
		'billing_limit' => '',
		'trial_amount' => '',
		'trial_limit' => '',
		'startdate' => current_time('mysql'),
		'enddate' => date("Y-m-d", strtotime( '+305 Days', current_time('timestamp') ) ),
	);

	// 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;
	}

	// Are they cancelling?
	if ( $level_id == 0 ) {
		// Check if they are cancelling level 5 or 6.
		global $wpdb;
		$last_level_id = $wpdb->get_var( "SELECT membership_id FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . $user_id . "' ORDER BY id DESC" );
		if( $last_level_id == 5 ) {
			pmpro_changeMembershipLevel( $downgrade_level_2, $user_id );
		} elseif( $last_level_id == 6 ) {
			pmpro_changeMembershipLevel( $downgrade_level_3, $user_id );
		}
	}
}
add_filter( 'pmpro_membership_post_membership_expiry', 'my_pmpro_membership_post_membership_expiry', 10, 2);