Success!
I called the box office today, right after they opened, and explained the situation, and they're going to let me exchange the tickets! I just have to go down there and do it in person, which is not a problem - I'm only ten minutes away from there. IF I had wanted to exchange for a different event, or return them altogether, it probably wouldn't have been possible. But just to change which performance of the same event, that they can do. Good to know.
All the more reason always to go through the box office if you possibly can. Ticketmaster would never have allowed it, and even if they had, they'd probably have kept the fee for the first tickets and made me pay it again for the second.
All the more reason always to go through the box office if you possibly can. Ticketmaster would never have allowed it, and even if they had, they'd probably have kept the fee for the first tickets and made me pay it again for the second.