Have a discount code only usable for old/expired members in PMPro.

oudc_pmpro_registration_checks.php

/*
	Old Users Discount Code (oudc)
*/
//define the discount code here
define('OUDC_DISCOUNT_CODE', 'abc123');				//change abc123 here to the discount code that is for old members only

//make sure only old members can use a specific discount code
function oudc_pmpro_registration_checks($okay) {
	//if something else went wrong, just return $okay
	if(!$okay)
		return $okay;
		
	//if using the oudc, check that they are logged in and an old member
	if(!empty($_REQUEST['discount_code']) && strtolower($_REQUEST['discount_code']) == strtolower(OUDC_DISCOUNT_CODE)) {
		global $wpdb, $current_user;
		
		//must be logged in
		if(empty($current_user->ID))
			pmpro_setMessage("The discount code entered is for previous members only. Log in to use that code.", "pmpro_error");
		else {
			$old_member = $wpdb->get_var("SELECT id FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . $current_user->ID . "' AND status NOT IN('cancelled')");
			if(empty($old_member))
				pmpro_setMessage("The discount code entered is for previous members only whose accounts have expired.", "pmpro_error");
		}
	}
	
	return $okay;
}
add_filter('pmpro_registration_checks', 'oudc_pmpro_registration_checks');

//give the code to old and existing users if discount code is empty
function oudc_give_members_code() {
	if(!is_admin() && !empty($_REQUEST['level']) && empty($_REQUEST['discount_code']) && is_user_logged_in()) {
		global $wpdb, $current_user;
		$old_member = $wpdb->get_var("SELECT id FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . $current_user->ID . "' AND status NOT IN('cancelled')");
		if(!empty($old_member))
			$_REQUEST['discount_code'] = OUDC_DISCOUNT_CODE;
	}
}
add_action('init', 'oudc_give_members_code');

//shortcode to show link to discount for old members
function oudc_shortcode() {
	if(is_user_logged_in()) {
		global $wpdb, $current_user;
		$old_member = $wpdb->get_var("SELECT id FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . $current_user->ID . "' AND status NOT IN('success', 'cancelled')");
		if($old_member) {
			//make sure the level id below matches as well
			return '
			<div><p>As a previous member of our site, you can use the discount code <?php echo OUDC_DISCOUNT_CODE;?> to save 50% on your renewal. <a href="' . pmpro_url('checkout', '?level=1&discount_code=' . OUDC_DISCOUNT_CODE) . '">Click here to renew with this discount code</a>.</p></div>
			';
		}
	}
	
	//return an empty string for existing members and non-members
	return "";
}
add_shortcode('oudc', 'oudc_shortcode');

This recipe is included in the blog post on "Offer Expired and “Old” Members a Membership Renewal Discount for Coming Back" at Paid Memberships Pro here: https://www.paidmembershipspro.com/offer-expired-old-members-membership-renewal-discount-coming-back/