Can I upgrade a Saver Day Pass

When is this trip?

Oops, I left out that important detail. I'm traveling the end of August into September.

Is it possible to upgrade at the last minute if there's not plenty of
space in 2nd class?

Yes. In the app choose your route: Zürich Flughafen - Montreux
You will see “tickets from, and an amount” click on that
Choose “point-to-point class upgrade”
If you know exactly which train you will be on, it might offer a Supersaver Class Upgrade, which will be valid for this departure time only - so don’t get it if you are not sure that you will be on that train.

It is also possible to reserve a seat on this train. If you want to you can reserve a second class seat for only 5.00 and no upgrade will be needed.

Thanks SwissNomad. I'll have to take another look at the site later today. Hopefully it will be as straight forward as it sounds.

So, I just realised that a seat reservation is actually not a good plan since it is also specific to an exact train/departure time and if your plane is delayed then you will miss it.

You can purchase the class upgrade at the last minute. They are always available. If you do go with the HFC, then be sure to get the half fare on your upgrade as well. I do think that getting the BO pass makes more sense with your revised itinerary.

Good to know: Supersaver Class Upgrades are often available up to the moment of departure. So once you know what train you will be on use the SBB app to buy one.

I'd been planning to get a 15 day Swiss Travel Pass, but now it seems like a Half Fare Card and a 10 day Berner Oberland pass makes more sense, doesn't it?

My gut reaction is yes as that is exactly what I would do.

Yes, the half fare card + reduced BO pass makes sense. The STP isn’t that great up in the mountains. Have a great trip!