You can use non-expired Amex vouchers when you make a new booking.
If your voucher has expired since you made your original booking you are allowed to rebook without a fee once, if the departure date is between 12 March and 17 August 2020.
For bookings within Norway domestic it's valid for bookings between 12 March and 15 July 2020.
Rebooking has to be done before the flight's departure through SAS Customer Service. Rebooking can only be done to and from the same destinations as the original booking, with a return date before 30 November 2020.