Not only would that not work, as others said, but you'd actually "taint" any additional funds you used to buy the new ticket, with the old expiration date.

For example, you buy a ticket for $200 on July 1, 2012. You then cancel that ticket. Those $200 expire on July 1, 2013. On June 15, 2013 you try this trick to extend the funds, by buying another ticket for $250, using the $200 credit and paying an additional $50 from your credit card, and then cancel the ticket. That *entire* $250 now expires on July 1, 2013. Any funds used to buy a ticket are given the *earliest* expiration date of all the funds used to buy the ticket.