I have Windows and Linux service nodes added already. When they were added for the first time I used administrator/root credentials and private keys.
Is it possible to change used administrator's credentials for service nodes?
You can safely change passwords on Windows and Linux service nodes.
Parallels Plesk Automation (PPA) uses these passwords only once for pleskd service installation on Service Node. After initial configuration, pleskd service is used for communication between PPA and service nodes and OS administrator's password could be changed freely.