Originally Posted by
JPDM
Rail has specific policies dealing with this. You can exchange for free for same day travel on the same route.
The same is more or less true for airplane tickets that are ~6折 and above (cheaper fares are less likely to offer penalty free changes), as long as the same fare class is available; when the fare class in question is sold out, you simply pay a fare difference. Fare class isn't an issue with trains because there is only one price.