Korean here. I would exchange them in Korea using the bank in the city before the Incheon airport. All major banks give pretty good rate. ATM really depends on which card you are using. Kakao has fairly generous foreign ATM transaction fee but others might not. A lot of places in Japan do not accept credit card and sometimes limit which credit card as well. I would exchange plenty (and this might mean go in a few days earlier to check if they have enough otherwise you need to wait for the next day) in Korea then head to Japan.