From Delta's Website:
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If I have purchased a ticket and a new fare is introduced, can I obtain a credit?
Yes, in many cases you may obtain a credit voucher (non-refundable tickets) or refund (refundable tickets) at any Delta airport ticket counter, City Ticket Office location, or over the phone (e>tickets only) for the difference in fare. A fee will apply for non-refundable tickets purchased for international travel.
When we introduce new fares or reduce a current fare in a particular market, you may change a ticket to the new fare if:
-the change is made prior to departure
-you do not change the flight, date, origin, destination or stopover points
-reservations are confirmed in the appropriate class of service
-all fare restrictions are met
-the lower fare is not a special delta.com only fare
Any applicable penalties are retroactive to the new fare ticket.