These look to pretty clearly be Economy Light/Basic Economy fares. Which means if you had booked through Delta, they wouldn't be changeable. Not sure where you got the MQD number. I see DL fares from $179 base/$400 YR which works out to 579 MQD and roughly $710 - $720 once you add taxes (similar to the KL fares). Given KLM"s current ticket policies for 2021, KLM looks to have been the better choice if you want to change them.