I'm afraid I don't have any helpful advice, just wanted to offer some more intel on return policy procedures, since the Ticketmaster and Harry Potter website make it sound so easy...
I have been buying tickets for my mom's birthday. I'm buying all the way out in December, which means there are actually a fair amount of options, particularly since I've been doing some strategic single seat purchases. My mom is quite tall and has some issues with restless leg syndrome due to chemotherapy years ago, so she really needs an aisle seat, and I'm not sure how much she'll want to do everything in one day, so I've been strategically buying tickets so that she'll have a few options. I bought the first pair in the first wave of general ticket sales, and since then have just been looking for other options, pairs and singles, each day.
The other day I found a good Thurs/Fri aisle option, but after finding the second seat, a better option came up and I bought that one as well, which brought my total to 6, with 3 tickets on one full performance and 1 ticket on another (+ my original pair). An aisle ticket came up on the 1 ticket performance, and so I wanted to return the worst of the 3 tickets and then get the aisle on the 1 ticket performance.
The return itself was actually quite easy - I did via live chat at like 11:30 at night, and about 36 hours later a full refund showed up on my card (fees and all, for those who were wondering about that!). Once I saw the refund and the ticket showed as returned on my account, I went to go buy the aisle ticket (still there!), and it was still telling me that I had maxed out my tickets.
I emailed Ticketmaster and they responded that "one of the conditions of returning your ticket came with the understanding that you will not be able renew your code". Now, before I started this crazy odyssey of buying a bunch of tickets of which I would return the majority, I read every single piece of information I could find on either website about the return policy, and the only limitations seemed to be timing of the return and that it could be at "box office discretion", but nowhere did it say that you wouldn't regain the ability to buy a different ticket when you return one. I've asked Ticketmaster to show me where this policy is, and if they can't demonstrate it, to tell me how to contest it, but I'm guessing that won't be successful (which is fine - I will still have a all-in-one-day vs two-consecutive-nights with aisle seats to offer my mom - just annoying).
At any rate, wanted to let anyone else who might have been hoping to return some tickets and get some new ones know that the 6 ticket cap may be applied in a very, VERY strict manner. (And if you can find the "no code renewal" policy, I'd love to see it!!).