That would be specific to FreeNAS, they changed the jails “engine” to iocage in 11.2, you might find more help in their forums/faqs
You could install the new plugin (which is iocage based) or just create a new jail, and move data over (if you plan on updating manually this is probably better anyway).
I’m not sure where the data is stored on the older jails, but the rc.d script should point to it.
@jeffdoto_gmail_com to clarify where did you find the data for you’re install?
here: ${JAIL_ROOT}/var/db/plexdata/?
or here: /usr/local/plexdata/?? (For iocage jails or plain FreeBSD this should be it although some user prefer to user a “flavour” port and have it in /usr/local/plexdata-pass
Also another hint in freebsd case (or freemas) is that if you have ZFS you can have this leaving in its own dataset and move/sanpshot as you want/need!
Since you moving it now, you could possibly create such new dataset, move the data there, then mount it on the jail.