Just as the title says, I have a travel voucher worth quite a bit, expiring in a couple of days. I don't really have a specific plan to travel soon. Can I buy a ticket for later, and change the date or refund it for credit later?
I don't know about the refund for credit later (others will know more about that). But you can (and I have) change it later for any trip of an equal or greater value, on any date within 11 months of *that* time, and it should be without penalty.
Yes, I've done it. Make it for 11 months from now. Experiment to get the correct amount or close to it. If you go over you will have to add new monies. Once you make the reservation wait a couple minutes and cancel it. You'll get a new voucher good for 12 months from today. By canceling that far in the future you won't be subject to any penalties. Plus, if necessary, you can push it forward again when it is close to expiring.
Yup. I have done it both at Amtrak and at United with flexible non-refundable fares. Works fine as there is no cancellation fee for a far out itinerary. On many airlines the non-flexible non-refundable fares have a fee for reusing the fare amount later. Not so at Amtrak yet AFAIR.