On Korean Air, you can change your flight date, but a fee may apply. The amount depends on your ticket type and whether it's a domestic or international route. But on average, the Korean Air Change Flight Date Fee starts from $40. Further, you might also need to pay the fare difference if the new ticket costs more. Besides, when you have purchased a flexible fare, the fee may be lower or even waived.

https://www.airlinestravelhelp.com/change-flight/korean-air-change-flight-policy/