no problems... i did it over the phone, spoke to a very sweet woman with a southern twinge to her voice. I fibbed and said I couldn't make the date, though I don't know that that mattered. When I got off the phone I even had a quite conscious thought that "that was so easy, and pleasant, and un-ticketmasterish".
I waited to buy my new tix until i got off the phone, so you may need a different excuse if you have both sets of tickets on the same account, though I don't think they would make a fuss about it.