American Airlines Change Fee are small fees charged to your account when you make changes to your flight. The fee is based on the difference between the original fare and the new fare. For example, if you originally booked a flight for $200 but then changed it to a $300 fare you would be charged $100 in American Airlines Change Fee.
Delta Airlines Change Fee is $200 per person, a maximum of $400 per reservation. The fee applies to changes made within seven days of departure and includes refunding the difference in fare if you choose a more expensive itinerary.
https://aviationrepublic.com/blog/american-airlines-change-flight-date-policy-fee-reschedule-booking/
https://aviationrepublic.com/blog/delta-airlines-change-cancellation-and-other-fees-all-you-need-to-know/