When existing devices are rediscovered, they may be classified as unknown.
If the existing devices are discovered as Unknown, choose the proper credential on rediscovery and they will be associated with appropriate device template, provided the SysOID of the device matches with any of the existing or custom device template. Rediscovering may result in modifying/updating some of the custom made changes.
Open the device snapshot page and click on the device that is marked unknown. Click on the top right menu and select Rediscover Device to rediscover the devices.