When I had to change (postpone) my ticket purchased with gift cards, Delta issued the entire amount as 1 voucher which expired within a year.

However, when I purchased a separate ticket with gift cards and had to cancel that ticket within the risk-free cancellation period (until midnight of the following day), the amounts were "reloaded" on the gift cards. So it's important not to throw away/delete the GC number AND THE REDEMPTION CODE until after the cancellation period has past. Delta does not keep a copy of the redemption code, according to my experience. So if you don't still have it when the GC "reloads," you'll lose the value as it will not be able to redeem it without the code.