Creates an OLDMEMBER merge field in MailChimp.

mailchimp_oldmember.php

<?php
/*
 * Creates an OLDMEMBER merge field in MailChimp.
 */
function my_pmpro_mailchimp_merge_field_oldmember( $merge_fields ) {
	$merge_fields[] = array(
		'name' => 'OLDMEMBER',
		'type' => 'text'
	);
	return $merge_fields;
}
add_filter( 'pmpro_mailchimp_merge_fields', 'my_pmpro_mailchimp_merge_field_oldmember' );
/*
 * Populates the OLDUSER merge field in MailChimp
 */
function my_pmpro_mailchimp_listsubscribe_field_oldmember($fields, $user) {
	global $wpdb;
	
	$has_membership    = pmpro_hasMembershipLevel( null, $user->ID );
	
	$new_fields =  array(
		"OLDMEMBER" => !$has_membership ? 'yes' : 'no', 
	);
	$fields = array_merge($fields, $new_fields);
	return $fields;
}
add_action('pmpro_mailchimp_listsubscribe_fields', 'my_pmpro_mailchimp_listsubscribe_field_oldmember', 10, 2);