Just for reference (so that future searchers will not be perplexed): according to the two reservation agents I spoke to today, be aware that you will forfeit the value of the canceled segments and have to eat a change fee.
In my case it was better to cancel the entire itinerary so as to better preserve the value of the original booking. Fortunately, the fares for the return segments hadn't changed much (if at all) since the original booking.
The lesson, it would seem, is to just book everything as a one-way.