Hi I Have booked a round trip ticket (bus) from Shinjuku to Kawaguchiko and I would like to ch...

This is just an additional information to the previous answer.

I would suggest booking another one way ticket from Kawaguchiko to Shinjuku while keeping your current round trip ticket. And then, when you pay for the ticket at the counter, please tell the clerk which one you would like to cancel. There are no discount applied to a round trip ticket for the bus from Shinjuku to Kawaguchiko anyway.

Hi there!
I think the most of companies accept the changes.
But if it's a packaged tour, maybe they don't.
You can ask via email if they have a contact address in their website.
If you have the receipt in English, check the policy once.

Hope this can be your help!