Add logic for downgrades

Author: annemirasol

includes/class-wc-subscriptions-coupon.php

@@ -222,7 +222,12 @@ public static function get_discount_amount_for_cart_item( $cart_item, $discount,
 			// Compute the sign-up fee. If it's a switch, we need to get the signup fee less
 			// recurring payment upgrade/downgrade costs.
 			if ( $is_switch ) {
-				$sign_up_fee = (int) $cart_item['data']->get_meta( '_subscription_sign_up_fee_prorated' );
+				$sign_up_fee_prorated = (int) $cart_item['data']->get_meta( '_subscription_sign_up_fee_prorated' );
+				$price_prorated       = (int) $cart_item['data']->get_meta( '_subscription_price_prorated' );
+				$is_downgrade         = 0 === $sign_up_fee_prorated && 0 === $price_prorated;
+				$sign_up_fee          = $is_downgrade ?
+										WC_Subscriptions_Product::get_sign_up_fee( $cart_item['data'] ) :
+										$sign_up_fee_prorated;
 			} else {
 				$sign_up_fee = WC_Subscriptions_Product::get_sign_up_fee( $cart_item['data'] );
 			}