Yes. But it's easy to do; just call and get a cancellation code. You'll need that if you plan to re-book (you can apply the tickets' value to the rebooking if done within a year).

That said, the hardest part of the process was the actual reissue of the rebooked tickets. Cancelling by phone was easy, and only took a minute or two. Rebooking by phone was as easy and as quick (I confirmed that our desired accommodations were available online before I called to rebook). But actually getting the reissued tickets (and paying a $12 fare differential) was a hassle; the station ticket agent didn't seem to know how to do it and had to make a couple of phone calls to get it done.