Just an update. Talked to 1K customer care. They called me back 1.5 hours after I called, and they offered to let us apply the full value of the ticket as long as it is still visible in their system (it may disappear any minute, will book today as soon as I have a chance). They even waived the change fee, as we had already paid it before. UA really didn't need to do any of this, especially waiving the change fee (since we did change the ticket to push her departure back one day before we cancelled). What an awesome airline. ^ :-:

Will post an update when I try to use the $$$.

Edited to add: Residual value applied to new itinerary, everything worked out great! :-)

One thing to add, it appears that at the airport when they adjusted my wife's itinerary last Christmas they did something quite strange, that elicited a "huh?" from each of the agents that looked at it over the last couple of days---something about converting the trip into two one ways where one of those retained the original ticket number---this might have been done to keep her return. I didn't know that was even possible. It was never fully explained to me. The moral of the story: it makes sense to ask questions to understand exactly what an agent is doing when adjusting a reservation, as this can have big time ramifications later on.