On my Ubuntu installation I use the recommended approach from @ChuckPa in this post
#on existing system
logout of Plex
sudo bash
cd /var/lib/plexmediaserver
tar cf /path/to/folder/PlexBackup.tar ./Library
#on new system
sudo bash
cd /var/lib/plexmediaserver
rm -rf Library
tar xf /path/to/folder/PlexBackup.tar
sudo chown -R plex:plex ./Library
sudo reboot