Hello Gift42
It depends. What kind of ticket did you buy? Did you also reserve seats for the Bernina Express?
Supersaver Tickets and seat reservations are not refundable and not exchangeable.
Saver Day Passes and Swiss Travel Passes can be used on both routes during the day of validity.
Point-to-point tickets: you can obtain a refund until the day before you travel and buy new ones.
Kind Regards
Danhiel