The H109 certificate (I believe that's what S and S+ members get as part of their "package") indicates that it's good for one free upgrade from BC to First or Coach to BC (on select trains).

The certificate indicates that it is "valid for one (1) one-class upgrade valid day of departure only and reserved no more than one (1) hour prior to train departure."

What exactly does that mean? Can the "whole" ticket only be bought one hour prior to scheduled departure? Or can I buy the base fare ahead of time and then go to the ticket window an hour before departure and attempt to upgrade my existing fare?

Thank you for help.