Change membership level when a user expires

only-set-default-membership-level-on-expiration.php

<?php
/**
 * Set a default membership level when a user expires (only). 
 * This does not set the default level when a user cancels.
 * Add this code to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function pmpro_set_default_level_only_when_expiring( $user_id, $level_id ) {

	pmpro_changeMembershipLevel( 1, $user_id ); // Change this to default level.

	return $user_id;
}
add_filter( 'pmpro_membership_post_membership_expiry', 'pmpro_set_default_level_only_when_expiring', 10, 2 );