As nsx alluded to, it may be possible to use the vouchers to book a flight, and then cancel that flight, effectively turning the voucher into travel funds. The funds would keep the same expiration, but they could then be reissued after expiration. Currently, you have 6 months from the date of expiration in which to request the reissue, and the new voucher created is valid for 6 months from date of issue. The fee is $100, which is deducted from the balance. And reissues are granted at the discretion of the agent, so they're not guaranteed.

I have no personal experience with this, so I'm not sure it would work. Hopefully someone else here can chime in with more definitive info.