I found that tarring up $PLEXJAIL/usr/pbi/plexmediaserver-amd64/plexdata, deleting old plex install/jail, reinstalling plex, recreating media mount point, and restoring the tar archive of plexdata within $PLEXJAIL/usr/pbi/plexmediaserver-amd64/ worked fine.
In my case, the gzip-compressed tarball was 3.3GB (tarball is 3.9GB uncompressed), and contained 388889 files and directories.
Note: using tar instead of cp preserves ownership and extended attributes, if any. There won't be any need to chown the directory as the previous example stated.
In my example, the ZFS volume used will be zfsvolume, thus the location for the jail would be /mnt/zfsvolume/jails/plexmediaserver_1. Your system will likely vary. Change the paths in the commands below to match your own use-case. To simplify, I'll reference this path in my examples as $PLEXJAIL.
1. Stop the Plex plugin from the FreeNAS GUI so that existing files/directories do not change during this process and open file locks are released.
2. From a root shell (via GUI, or ssh + "sudo su -"), create a tar archive of the Plex database directory. I used the following approach, which took 10-15 minutes to complete (you can use bzip or xzip, but I wanted a known-fast compression over smaller):
cd $PLEXJAIL/usr/pbi/plexmediaserver-amd64/
tar cvfz /mnt/zfsvolume/plexdata.tar.gz plexdata
3. Close the shell (or cd out of the jail so that deleting the jail does not fail)
4. Delete the existing Plex plugin using the FreeNAS GUI.
5. Reinstall Plex using the FreeNAS GUI.
6. Recreate the media mount point in the newly recreated Plex jail, pointing to the /media directory within the jail (see Plex install instructions for this).
7. From a root shell, restore the tar archive. I used the following approach (restoring is much faster than archiving):
cd $PLEXJAIL/usr/pbi/plexmediaserver-amd64/
tar xvfz /mnt/zfsvolume/plexdata.tar.gz
8. Start the Plex plugin from the FreeNAS GUI. If you experience the 500 error in FreeNAS 9.2.0 like I do, just reboot the server before starting Plex.
9. Verify that everything is working by accessing the Plex UI. Existing media and preferences should be present. Once confirmed, delete the backup tar archive from a root shell (remember, root owns it):
rm /mnt/zfsvolume/plexdata.tar.gz