|I reached Incheon Airport around 7am and went to K Books to collect my EG SIM card.|
It is located at the Arrival Hall near exit 7.
"GS25 포스 충전 방법 : 서비스 상품>편의점 캐쉬>알뜰폰 선불 충전>이지카드 바로 충전>동글이 충전"
By the way, you can only transfer your money from voice data to data plan only when your data balance becomes zero MB. You can only transfer if you have enough money in your voice data. Of course, you can also choose to terminate your data plan before it hits zero MB. As shown in the picture below, there is not much balance left in this case. Hence, it is up to individual whether you want to terminate the data plan and transfer the money from voice data to data plan or wait for it to become zero. Once you terminate your data plan, you can transfer the money from voice data to data plan.