Hello Fooyejoak47
Sorry @Tafunge25 , your answer is not exact for Italy.
No problem in Switzerland, you can travel in 1st class with or without reservation.
But, in Italy, you need a mandatory seat reservation. If you also want to travel in 1st class in Italy, you have to exchange your 2nd class ticket. I don’t know if a class upgrade is possible or if you have to cancel your 2nd class ticket (unfortunately with a 20% fee) and buy a new one in 1st class. Discounted tickets may be not refundable or not exchangeable.
I suggest to contact SBB by phone or chat, look at the bottom of this page:
https://sbb-tickets.cc/en/help-and-contact.html
Kind regards
Danhiel