I recently bought a domestic ticket on united.com. Paid about $50 via CC and $250 via a customer service e-cert (i.e., total cost $300). Looks like I need to change the flight. Will I lose the value of the certificate in the process, i.e., will I end up with a value of $50 and pay a $150 change fee? If not, how does the expiration date of the e-cert figure into this situation? It was "use/fly by May 8" and my new flight will likely be past that date. Thanks!