Long story but my router died, got a replacement to find it wouldn’t support the 10.0.0.0/8 subnet for private IPs.
So I had to rush in a re-ip. My Plex libraries are UNC paths to a NAS. Perhaps brashly I thought I’d just edit the path to the new IP. Now it’s rescanning everything and it’s lost all my watched data
Once the conversion is done, you can do as you please.
Although you should avoid enabling both library update options at the same time.
And you should never enable 2) if your media files are not stored inside the server machine itself.
Thank goodness for the auto backup - there where three monthly ones there so I managed to restore from a backup in May, much better than loosing years of watch history! Then I just edited the DB in sql lite to tweak the paths and everything was Ok.