Successfully completed rebuilding jails/ on my machine this weekend. This should have been much easier for plugins, but was what I expected for my standard jails.
1) Re data management
I wasted a lot of time thinking that there should be some clever way to use clones to make copies of my datasets under jails. This could have been an OK idea, but I thought I would try the GUI and learned that it is hostile to making multiple clones at once and does not have an support promoting clones. I recommend making a copy or two using old fashioned rsync and maybe tar after stopping the jails.
I then deleted the plugins followed by the jails followed by the jails/ dataset and the child datasets that I had backed up.
2) Then I added a transmission plugin. All the Jails - Storage mappings I had were still there, but I had to edit each one for the new jail name. At this point I realized I had not changed the Jails - Configuration to point to the 9.10 Collection URL, but it appears not to have mattered. I copied the jail settings including mac addr, and finally I copied over var/db/transmission from my backup. Restarted the jail and then started the plugin and everything just worked. (yay!)
I did the same thing for plexmedia server, copying over var/db/plexdata from my backup, but it didn't work. :/ I also copied usr/local/share/plexmediaserver and it still didn't start after restarting the jail or trying to control through plugin gui. Nothing in "var/db/plexdata/Plex Media Server/Logs", either. But after trying to restart another time with the rc script worked. Plex now starts up when I restart the jail, and my library metadata is all intact. But the plugin ui has still been showing service status off.
¯\_(ツ)_/¯
Next I manually entered a Template to look like
http://doc.freenas.org/9.10/jails.html#managing-jail-templates. I'm not sure if this is necessary.
Then I re-created one of my standard jails for running smokeping. It took a little trial and error to install all the packages needed for the binaries smokeping uses and a config change to update the location of dig. Only negative is that my smokeping metrics have a ~day gap while I was screwing around with this stuff.
3) Caveats worth noting
a) Before you delete your jails:
* run 'pkg info' to get a good list of packages when you are rebuilding.
* use rsync to make a backup after quiescing all services / stopping the jails
* take some screenshots of the jail configs in the web ui. The individual jails, the jails storage, and the jails config
b) Don't delete the Jails - Storage mappings, these can be updated with the new jails' names.
c) Update the Jails - Configuration Collection to point to the 9.10 Collection url "
https://download.freenas.org/jails/10/x64/", but it probably isn't important.
d) Maybe create a template under Jails - Templates using url:
https://download.freenas.org/jails/10/x64/freenas-standard-10.3-RELEASE.tgz and mtree:
https://download.freenas.org/jails/10/x64/freenas-standard-10.3-RELEASE.mtree