Here is the latest from Delta:
For international travel originally scheduled to depart March through September 30, 2020, all change fees are waived.
You can rebook your trip to the same destination for travel departing before September 30, 2020, with no difference in fare applied.
For trips rebooked to the same destination for travel departing October 1, 2020 – September 30, 2022, there will be no change fees. If there is a difference in price, the fare difference will apply.
If you’d like to change your destination, or if you don’t know when you want to travel, you may cancel without a fee and use your eCredit to book a new flight at a later date.
Eligible Customers: All Tickets Originally Issued on or before April 17, 2020
New Ticket Must Be Re-issued On/Before: September 30, 2022
Rebooked Travel Must Be Completed No Later Than: September 30, 2022