Delta Airlines has announced that it will no longer charge passengers to change flights within the United States or to foreign flights departing from the United States. If your new flight ticket is more expensive, you'll simply have to pay the difference in fare. The charge waiver, however, does not apply to Basic Economy tickets, which cannot be altered or cancelled unless they are covered by Delta's Basic Economy Changeability Waiver for 2021. (explained below).

In some circumstances, Delta Airlines' flight modification policy allows you to change your flight (date, time, or destination) or seek for a refund in the form of an eCredit.

Below is a snapshot of Delta Airlines flight change policy

Until December 31, 2021, Delta Airlines has waived the change charge for Basic Economy tickets (inclusive). Although Basic Economy tickets are typically non-changeable, you can make adjustments to any Basic Economy flight ticket during this time without incurring a flight change fee.

You've got options. You can rebook your ticket right away if you know when you'll be travelling. Alternatively, you can cancel it and receive a full refund, rebook it by December 31, 2021, and travel by December 31, 2022. If your new flight is less expensive than your old one, you will be given a credit for the difference in fares, which you can use for future travel.

https://flyamericans.com/blog/delta-airlines-flight-change