Determine member's previous level and assign them a new level upon cancellation

pmpro-upon-expiration-change-membership-levels.php

<?php
/**
 * After expiration, assign them a specific "cancelled" level.
 *
 * Can be used to downgrade someone to a free level when they cancel.
 * Will allow members to the "cancel level" to cancel from that though.
 */
function pmpro_upon_expiration_change_membership_levels( $level_id, $user_id ) {
		// set this to the id of the level you want to give members when they cancel
	$last_level_5 = 5;
	$last_level_6 = 6;
	$level_5_cancel_id = 2;
	$level_6_cancel_id = 3;

	// 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;
	}

	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_5 === $last_level_id ) {
		// otherwise give them level $cancel_level_id instead
		pmpro_changeMembershipLevel( $level_5_cancel_id, $user_id );
	} elseif ( $last_level_6 === $last_level_id ) {
		// otherwise give them level $cancel_level_id instead
		pmpro_changeMembershipLevel( $level_6_cancel_id, $user_id );
	} else {
		return; // let them cancel
	}
}

add_action( 'pmpro_membership_post_membership_expiry', 'pmpro_upon_expiration_change_membership_levels', 10, 2 );