Hello ABooth
Is it a Point-to-point ticket or a Supersaver ticket? If it is a Point-to-point ticket, it can be changed at the travel center, or even the conductor can change it. If it is a Supersaver ticket, it cannot be changed, and you must purchase a new ticket if you wish to travel via the panoramic route.
It is not advisable to book tickets with third-party agencies. Next time, simply purchase from SBB directly: sbb-tickets.cc/en.
Kind regards,
Roland