Originally Posted by Often1

There is some just plain lousy advice in this thread. It's fine to have an opinion as to how one wants things to be. But, suggesting that it is the way one wants it to be does not help people who really have a practical answer and want a legala nd practical answer.

1. If you want a fully refundable ticket, purchase one. They are more expensive and most do not, but it is a matter of personal preference.
2. If DL cancels or significantly changes your flight, you are entitled to a refund and need not accept alternatives it offers, including reroutes and credits. This has been DOT policy since 2011 and was reinforced by two Warning Letters issued by DOT to carriers in March and April of 2020 as the pandemic took hold. Nothing remotely suggests that you can have a do-over and, having accepted a credit, come back and have a refund. If DL wishes to grant this as a customer service gesture, it is free to do so. But, any suggestion that one may have this as a matter of DOT rule or contract is simply incorrect.
3. Those who suggest that DL has made a contract to fly you on a specific date and at a specific time, have failed to read the contract.

What OP is asking for is the benefit of an earlier lower fare 6 months later. Not a rebooking in IRROPS.

As a general matter, accepting a credit is a lousy deal. Not only are you making an interest-free loan, but you limit yourself in the future. Perhaps some other carrier has a better deal, routing, hard or soft product which was not available 6 months ago. With a refund, one simply purchases the new ticket on the new carrier. Or perhaps, now is not the time to travel, so the refund goes into the bank or into a new washing machine or whatever.