Basic Economy award fares are non-changeable and non-refundable. Where do see Delta claim they are refundable? You can see they specifically excluded them in announcement below. They may have allowed it on tickets purchased on/before April 30, 2021 when revenue basic economy fares were still changeable due to Covid.

https://news.delta.com/delta-extends...y-award-travel

Also, they exclude Basic Economy awards here on redeposit fee waiver --

https://www.delta.com/us/en/travel-p...ng-made-simple


SkyMiles Members can also now enjoy greater flexibility and peace of mind when traveling on an Award Ticket with the ability to change their plans at any time.

The following changes are effective immediately for Award Tickets for travel originating in North America to anywhere in the world, excluding Basic Economy tickets:

• Award Ticket redeposit and reissue fees have been eliminated

• Members may change or cancel Award Tickets before departure. No longer will changes and cancellations made within 72 hours of departure result in the loss of miles*

*For tickets purchased after August 9, 2020.