Hello Soli13
A change is not possible; you would have to purchase two new Supersaver tickets at half fare for the same day / route / passenger, and ask SBB to cancel the full fare Supersaver tickets afterwards while declaring the ticket numbers of the new tickets. The cancellation fee is CHF 10 per request.
I doubt it will be worth the hassle.
https://www.sbb.ch/en/help-and-contact/refunds-compensation/refunds-compensation/ticket-refunds.html
When being asked for ticket advice, we always suggest to finish the complete Swiss trip planning and only then to decide on the tickets by comparing the different ticket offers and calculating the ticket costs in a spreadsheet. This will give you the rough idea of whether to buy the Swiss Half Fare Card or the Swiss Travel Pass, or any regional pass. The purchase of the tickets should be the last step of your planning to avoid extra costs.
Furthermore, when arriving by air, or for sightseeing trips, Supersaver tickets are not recommended as they do not provide any flexibility. You must stick to the connection on the ticket; if you miss the first departure, you are required to purchase a complete new ticket at normal fare.
Kind regards,
Roland