functions.php

<?php

// Quick and dirty *AUTOMATIC* upgrade-only discount for EDD (no discount code)
class SearchwpUpgradeDiscount {
	private $upgrade_discount = 30; // 30% discount
	private $applicable = false;

	function __construct() {
		add_filter( 'edd_get_cart_content_details_item_discount_amount', function( $discount, $item ) {

			if ( ! empty( $item['options']['is_upgrade'] ) ) {
				$this->applicable = true;
				$discount = floatval( $item['options']['cost'] * ( $this->upgrade_discount / 100 ) );
			}

			return $discount;
		}, 10, 2);

		// Output a note next to the Total to reiterate the discount
		add_filter( 'edd_cart_total', function( $total ) {
			return $this->applicable ? $total . ' (upgrade discount)' : $total;
		}, 10, 3 );
	}
}

new SearchwpUpgradeDiscount();