If, at some point, if I upgrade FreeNAS to 10, when it comes out, will any jails be automatically upgraded?
Are you asking if the jail template gets updated to support FreeBSD 10 jails should you want to create new ones or if the currently installed jails/plugins get updated to the newer version?
I'm only speculating here, I have no formal knowledge however I would think the jails that were in existence before the upgrade would remain where they are at. Also any new jails would require the new jail template to be downloaded which would be compatible with FreeBSD 10.
Now that is if you do not consider that with FreeNAS 10, as far as I understand there will be a game changer in town, a new VM application called Bhyve and with all hopes it will be a fully featured VM environment. Again, I have not been keeping up with the new FreeNAS 10 development so I could be way off base here, I just hope not.