Yes, they can be reopened for canceled flights.
- Flexibility: If you have to change or cancel a reservation where an Upgrade Certificate has already been applied and confirmed, you will not lose the Certificate. You can either call Reservations to apply the Upgrade Certificate to your new flight, or it will automatically be reopened in your account within 2-3 weeks.
https://www.delta.com/us/en/skymiles...es#certificate