Thank you for your request.
Yes, it’s possible to change to annual payment. There’ll be a change of your contract instead of a new subscription. If you change the GA to annual payment, it is valid for one year from this date. You can do so by calling the SBB Contact Center: 0848 44 66 88 (CHF 0.08/min). You could also go to a staffed ticketcounter and ask for the change there.
I wish you a lovely day.