Hello Ghueyoilouk66
Welcome to the SBB Commnunity.
GA 1-month travel cards are destined for residents having a Swiss Pass card. Tourists can purchase it as well, but the card will be a paper ticket; no SwissPass card is needed. An SBB account is helpful to purchase tickets online, though.
But: why would you want purchasing the GA travel card on the 16th only, while you arrive at the 15th and the SBB ticket desk is open until 9.00 PM (21.00)? This does not sound reasonable to me.
To answer your question: YES, it is possible to refund tickets for the 15th and have the GA travel card’s first date of validity back dated to the 15th under the sole condition that your ticket for the 15th bears your name. Unpersonal tickets from the machine are not accepted for refund unless the use was previously confirmed by the train personnel by a special attestation.
Kind regards,
Roland