Hi ikinpandai
You can buy a point-to-point class upgrade. See www.sbb.ch/en.
Enter the start and end destination, choose the Bernina Express connection, click buy and select the option Point-to-point class upgrade.
The cost depends on the journey chosen. For example, Chur to Tirano is CHF 48.
The Day Class Upgrade you can buy provided you are in possession of the Swiss Half Fare Card.
Best regards,
Roland