The ticket you buy with your voucher will clearly state that the amount you paid with your voucher is non-refundable.

The only way is to change such a ticket later on - but the agents are supposed to document the reservation accordingly to avoid that. So the gamble is yours.

Please note that this does not apply to Transportation Credit Vouchers (TCVs) issued by customer care. Those only need to be ticketed by the expiration date, and tix can be changed within the 1 year validity according to the particular fare rules.

Maybe you want to try to swap your DBC voucher for something of equal value to you in the Coupon Connection forum.