Connect the old Nucleus Plus back onto the network, USB keyboard and, by HDMI to a monitor or TV. When it powers on it should display an ipaddress consistent with your network (same subnet).
There is a possibility that, if your old Nucleus was configured to use a static ip address, that ip address will conflict with the ip address of another device on your network - e.g. your new Nucleus One.
If that is the case, or if you are not sure, follow the procedure details at:
Note: RoonOs is the operating system run by Nucleus and ROCK devices.
You can then use a web browser on another device to navigate to http://nucleusplus (or if that doesn’t work http://<ip_address> where <ip_address> is the one displayed on the monitor connected to the old Nucleus Plus).
From there you should:
Reset the database
Format any internal storage.
Since you have already logged into your new Roon Server (and thus de-authorized the old one), your old Nucleus Plus should be ready to pass on and will connect to the new owners network just as a new Nucleus would.