I made a seat reservation for the Glacier Express from Zermatt in first class and paid for it. I have a Swiss Travel Pass that is second class. Is this ok? Do I need to do anything else?
You will need to buy an upgrade ticket, basically the price difference between a 1st class and 2nd class ticket.
You do not need to hurry to get the upgrade though. You can easily get that at the station. Price will be the difference between 2nd and 1st, but with the Half Fare Card reduction applied.
If you are going all the way to St Moritz, that will be 116 CHF per person.
FWIW, I just watched an hourlong special on the GE on the PBS app on my Roku. It seems to have been a Nat Geo production originally.
https://www.imdb.com/title/tt25436306/episodes/?season=1
Well done and informative.