What jhayes_1780 says is correct. Generally the only way you can get an e-cert replaced is if the reason you cancelled your ticket is UA's fault..or perhaps if you have some extraordinary story to tell them about why you need to cancel.

I used one for a trip to New Orleans in 2005..but Katrina hit a few days before I was supposed to travel so all flights were cancelled. UA refunded the ticket and I contacted customer relations and they replaced the discount certificate (it was actually paper at the time..e-certs weren't used as much back then).