Yufeemau63
A question to be sure if I understand you correctly:
Did you pay the 1st class seat reservation 49 CHF or more?
The price of the upgrade between Zermatt and St. Moritz is 113 CHF, less if you don’t make the entire trip.
An Interrail Pass adult for 3 days within 1 month costs 165 € in 2nd class, 210 € in 1st class. It is cheaper to buy an 1st class pass instead of an upgrade.
(All prices without guarantee of exactitude)