If you back up the registry, then yes. Your shared users are safe.
Make sure to shut down the Plex server when you copy the Plex data folder. Because if you don’t, you’ll end up with a damaged primary database file.
If you have a second (preferably internal) hard drive, I recommend you to move the location of the Plex data folder to this second hard drive first: [HowTo] An extended guide on how to move the Plex data folder on Windows
Because then, all you have to do after a Windows reset/reinstallation is to double click the saved registry file and then install Plex server and be done.
