Dan6 said: "I've successfully done what you asked a couple of times - purchased tickets, later got a discount code, and then did an exchange through calling Telecharge (technically a new purchase followed immediately by a refund of the old one) for better seats using the discount code. On one occasion, they even let me repurchase the same seats I had previously bought at a lower price using the discount code, but I view that as a fluke from a customer service rep who was just having a very good day."
Thanks for the valuable info. The one time I did the exchange, I did exactly what you described: Called Telecharge, the phone agent immediately indicated no problem, just buy the new tickets and we'll cancel the old ones. But really glad to hear that you had success in purchasing new, better seats for less money using a discount code. And yeah, I would bet that repurchasing the exact same seats at a lower price was a big-time fluke, glad you were able to pull it off.