- Stop the primary instance and detach it's volume.
- Spin up a new instance with the security groups and availability zone.
- Create a new keyfile for the dummy instance (or choose an existing key that you do have).
- Attach both volumes to the dummy instance.
- Ssh into the dummy box and copy the authorize_keys from the dummy volume to the primary volume.
- Stop dummy instance, detach volumes, and reattach primary volume to primary box.
- Login to the primary instance using your keyfile!