I have a set of VDB vouchers ($400) expiring later this week. (forgot that I had them). Can I use them for 2 refundable one way tickets and later use each of the new tickets for another ticket. I was thinking of using them for LAX-SFO & SFO-LAX H fares for June 2012 travel ($203 each way--$3 on CC and $200 voucher). Would I be able to exchange later each ticket for another fare that is more expensive (say $220ai and pay the $17 difference for each ticket? I really want to extend my vouchers and would the above work?