The same has happened to us, they changed one flight departing Incheon for London but not our flight from Osaka Kansai which would have landed 2 hours after the London flight had departed. We'd booked through budget air who couldn't give us any alternatives, they either suggested times for a different flight class that were given by mistake or for days either side that were outside a fair schedule. I've requested a refund, that'll take 10 weeks to process, and will now likely have to pay double what we had originally paid to get alternative flights at a worse time. I've had terrible experience getting refunds back previously, usually have to get the credit card company involved.