Allows a custom renewal discount to be set for customers who purchased before a custom date
<?php
/**
 * Plugin Name: Easy Digital Downloads - Grandfather Renewal Rate
 * Description: Allows a custom renewal discount to be set for customers who purchased before a custom date
 * Author: Pippin Williamson
 * Author URI: https://easydigitaldownloads.com
 * Version: 1.1
 */
/*
 * Sets renewal discount to 40% for any customer that purchased before April 18, 2016
 */
function pw_edd_grandfather_renewal_discount( $renewal_discount, $license_id ) {
	$license = edd_software_licensing()->get_license( $license_id );
	if( strtotime( $license->date_created ) < strtotime( 'January 1, 2020' ) ) {
		$renewal_discount = 40;
	}
	return $renewal_discount;
}
add_filter( 'edd_sl_renewal_discount_percentage', 'pw_edd_grandfather_renewal_discount', 10, 2 );
To implement this snippet on your site, create a brand new, empty WordPress plugin and paste the above code inside.