I booked a round-trip ticket, but if I need to book a different date for my return due to circumstances, do I lose the original return ticket?

For example, if I booked a round-trip ticket to China for 300,000 KRW...
I flew out from Incheon Airport as planned, but if my schedule changes and I need to return 3 days later,
do I lose the original return ticket to Korea? Or can I get a partial refund?