Change and Cancellation Policy

Hi, Id like to buy online the train tickets G6586 for December 3. I will arrive with a flight at Hong kong. Could you let me know if there is the possibility to change the ticket (in case of delay) for free or the fee for the change. The cancellation policy terms? Whan I purchase the ticket online the ticket is immediately issued or is it issued when I collect at the station? Thank you in advance. Regards