Wughuquoos68
For some reason, the answer for your very Glacier Express specific question is missing in the English version of the FAQ on the Glacier Express website, only in the => German version.
Here’s the translation:
I have a 2nd class ticket but would like to travel in 1st class, how do I pay for the class upgrade?
You can make the reservation in 1st class. In step 3 under Reductions you can choose the Swiss Travel Pass, so you only pay for the reservation. The difference between 1st and 2nd class can only be paid at the station. How much the class change costs depends on the route.
The usual class upgrade tickets appear to be unavailable in the app or online for the Glacier Express.