Assuming you booked before May 15, 2020:
The August 31st 2020 rebooking deadline is only if you want to have the 50EUR/CHF/USD extra value.
Otherwise, if the flight is not canceled yet, you can change once free of charge, up to January 31, 2021 for a new date by December 31, 2021.
https://www.swiss.com/us/EN/various/...ng-coronavirus