Hello tyan112
Yes, this is possible. On routes that are fully covered by the Berner Oberland Pass, you can buy a class upgrade at half fare. The easiest way to do so is using the SBB Mobile App, by declaring the Half Fare Travelcard in your passenger details. Simply click on the pen, to modify your passenger data.
Our SBB Community has an English-speaking platform, too: Questions and answers - SBB CFF FFS Community
Kind regards,
Roland
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