Hello Bless
Welcome to the SBB Community.
Although your question has been answered multiple times, I will do again 🙂
Yes, the Class Upgrade for a Day is available for Swiss Travel Pass holders, too.
You can easily purchase the upgrade online, for example with the SBB Mobile app (tab Shop), that you should download in any case.
You could also get it at the RhB station but no need to do so.
The Supersaver Class Upgrade is for one single connection only. It is fine if you can be sure to make the connection. It is available at ½ fare, as any class upgrades for a route with the Swiss Travel Pass. Simply select ‘½ fare card as the discount’ type.
And, kindly note, the train is named Bernina Express, named after the Bernina mountain ridge.
Kind regards,
Roland