Place a PMPro member in another level when they expire and set an expiration date.

my_pmpro_after_expire_membership_level_default_level.php

<?php
/**
 * When users expire (and the system changes them to membership level 0) we give them another downgraded level.
 */
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_1 = array(
		'user_id' => $user_id,
		'membership_id' => 1,
		'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( '+ 1 Year' ) ),
	);

	// 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 from level $cancel_level_id.
		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 == 1 ) {
			return;	// Let them cancel.
		}

		// Otherwise give them level $cancel_level_id instead.
		pmpro_changeMembershipLevel( $downgrade_level_1, $user_id );
	}
}
add_filter( 'pmpro_membership_post_membership_expiry', 'my_pmpro_membership_post_membership_expiry', 10, 2);