When keeping your ticket to use its value towards a future itinerary, be aware that UA may have problems finding it again.
When I had to cancel a trip because Mrs 1P was having major cancer surgery, there was no refund, just an unused ticket. But it took UA no less than 3 weeks to locate the ticket number in their system when I wanted to use it several months later.
This is a good example of why you should always keep ticket numbers until it is completely used. Since it's in the email receipt you get, it's easy to keep in a saved email folder. On the other hand, of course, really my, the unused itin should theoretically be available in your MP profile.