Change user's membership level only when a user expires from a specific level.

change-level-only-when-user-expires.php

<?php
/**
 * Change membership level for users expiring from startup level, moving them to primary level.
 * Users can cancel their membership level after their before or after the membership level change.
 */

function wbon_change_default_level_when_expiring_startup( $user_id, $level_id ) {

	global $wpdb;

	$SQL = "SELECT membership_id FROM $wpdb->pmpro_memberships_users WHERE user_id = $user_id AND status='expired' ORDER BY id DESC LIMIT 1";

	$results = $wpdb->get_results( $SQL, OBJECT );

	$expired_level = $results[0];

	$old_level_id = (int) $expired_level->membership_id;

	if ( 3 == $old_level_id ) {
			pmpro_changeMembershipLevel( 2, $user_id );
		}
	
	return $user_id;
}
add_filter( 'wbon_membership_post_membership_expiry', 'wbon_change_default_level_when_expiring_startup', 10, 2 );