I unregistered my device from my account but I can't register it to a different account

When you unregister a device, make sure that you have a stable internet connection so that the change can reflect in our database. This can take up to 10 minutes depending on your internet connection and mobile device. 

Once some time has passed, try to register the device to the other account again to validate that the database has updated, freeing up your Rad device for a new registration.