Prevent Discounts on Upgrades

function pw_edd_disable_discount_on_upgrade( $return ) {

    $cart_items = edd_get_cart_contents();

    if ( $cart_items ) {

        foreach ( $cart_items as $item ) {

            if ( isset( $item['options']['is_upgrade'] ) ) {

                $error_message = __( 'Discounts are not valid when upgrading.', 'edd' );

                edd_set_error( 'edd-discount-error', $error_message );

                return false;

            }

        }

    }

    return $return;

}

add_filter( 'edd_is_discount_valid', 'pw_edd_disable_discount_on_upgrade', 99, 1 );