You can always call DL and ask for fully refundable Y or J

Or just book cheapest ticket shortly before your outbound flight and use 24hr free cancellation.

One advantage of booking a roundtrip (and paying a change fee later) is that your new ticket is priced on historic fares on the day your original purchase, subject to booking class availability. Even if you depart from different city or your original booking class is not available, you are still paying the cheapest historic roundtrip + change fee.