Thanks to this thread, I was able to change a $20 paper travel voucher from November 2011 (expiration Nov 12) to an e-cert. And, happy to discover, the expiration date got changed to August 2013, a year from now.
Here is the link:
https://secure.unitedmileageplus.com...77391D21A06493
Otherwise, I typed in
www.united.com/papercertificate
Thanks to FT !
Doris