Hi Dirk,
I have tested this on my lab server and it has worked for me. You need to select the device that is in the Custom Device Family and right click and select Remove. Remove will only remove the device from the custom device family but the device will not be deleted.
Once the device has been removed you will need to find the device and right click and select Reconfigure, Reevaluate NCM Device Family and the device should be placed back into the Cisco IOS device family.
I hope this helps.
Best regards,
Glenn