SB4
May I ask what kind of 2nd class ticket do you have and which you would like to combine with a class upgrade? Is it a point-to-point or a day pass or a travel pass? Do you have the Half Fare Card in addition?
The point-to-point class upgrade is only valid on the specified route, as indicated on the ticket.
If your class upgrade reads Geneva-Wengen VIA Bern-Spiez-Interlaken Ost, you can change trains along the way where ever you want, say either Bern or Spiez. But, the upgrade would not be valid via Montreux-Zweisimmen-Spiez.
A combination of Super Saver tickets and Super Saver Class Upgrades isn’t possible, either.
If you allow a comment concerning your trip: check the routing via Montreux-Zweisimmen-Spiez, the so called “Golden Pass Panoramic Express”. A very scenic train line, well worth to be travelled.
Best
Roland