Being on the server machine itself, on the plex Dashboard page, I was able to change password via the top-right account icon → Account Settings, and then change password, WITH that box ticked (that logs you out of all sessions).
Then a few panicked minutes of re-launching the server, seeing none of my libraries, and being told in the Dashboard page that my server was unclaimed. There was a button to Reclaim the server… and all seems OK now after another re-launch… good luck everyone… yikes. Thanks Plex! Secure your shizz