Upgrading seats with Swiss Travel Pass

I don’t know the answer to your question but I do know every second class train car I rode in Switzerland last year was comfortable, clean, and spacious. From what I hear that is the one country that it’s really not worth it to upgrade.

I'll go farther and say every 2nd class seat I've been in for all my trips to Europe was clean and comfortable. For Switzerland, my cavaet would be if you do any of the scenic trains, it is probably worth it to reserve a spot in a scenic car.

There aren't that many long train rides in Switzerland to start with, and last October when I was traveling on the 2nd class Swiss Travel Pass many times I had whole cars to myself. It's too easy with the Pass, just hopping on and off -- or changing your mind which time you want to leave -- to mess with 1st class seats! If you don't like what you find in one 2nd class car, you just walk down into the next one. Also, if you haven't already found the SBB App for your smart phone, it's the best!

I agree there are no long train rides in Switzerland. But there is way to buy 1st class as you go.
You have to go see an agent at the station. Also I had a 2nd class Swiss Travel Pass and was able to buy 1st class on the Golden Pass Line while on the train. The conductors all have credit card machines as they know tourists multiple languages from all over the world are confused about the rules. But they make money off that

In most cases I would not upgrade. Rides are on the short side, and for longer rides, often there are transfers involved that break the journey. I can't think of any "typical tourist" one-seat ride that exceeds 3 hours.

The price to upgrade (buy in station) is the price difference between a 1st and 2nd class ticket on a given route, usually about half the price of a 2nd class ticket.

Just get on the train, choose your 1st class seat and wait for the conductor to come check your ticket. Tell them you would like to pay the fare difference. Alternately if you pass a conductor while boarding just go ahead and tell them at that point

Just get on the train, choose your 1st class seat and wait for the conductor to come check your ticket. Tell them you would like to pay the fare difference.

I don't think you can do that. Train staff no longer sell tickets, you must buy your tickets before boarding the train.
If you want to upgrade to 1st class (which I also consider unnecessary), you need to buy an upgrade ticket.

Information on upgrades here: https://www.sbb.ch/en/travelcards-and-tickets/tickets-for-switzerland/class-upgrade.html

You can always decide spontaneously whether you would like to travel in 1st class. Buy a Route-specific Class Upgrade in the webshop, via the smartphone app, at ticket machines or at the ticket counter.

Just get on the train, choose your 1st class seat and wait for the
conductor to come check your ticket. Tell them you would like to pay
the fare difference. Alternately if you pass a conductor while
boarding just go ahead and tell them at that point

Be careful, this no no longer allowed and you may be fined for doing so at the discretion of the inspector.

Furthermore there are now restrictions on when you can buy the upgrade on the mobile app as well. The purchase must be made before the train leaves the stations.

Yep, you’re right. I remember that now. So get the SBB app and purchase an upgrade via your phone.