Hello A_B
Thank you for your message.
You can change your contract at an SBB Travel Centre or by calling the SBB Contact Centre on +41 (0) 848 44 66 88 (CHF 0.08/min.). Switch from annual billing to monthly payments.
Credit balances, e.g. from deposits, can be credited if you switch immediately (switch today). If the switch date is in the future, the amount cannot be credited and you will receive a payout.
Have a nice day.
Best regards, TatianaR