I understand about the backup. Seems like sage advice, but I'm tapped as to resources I'm willing to donate to this project.I said it before - "ZFS is *NOT* a backup of your data!".
Before doing *ANYTHING* backup the data to storage that isn't connected to FreeNAS.
Even if you screw up and blow up the pools so you have to redo your shares/permissions/jails/etc. at least the data is intact.
Since you are running 4x 1TB in RAID5-style (I assume it is RAIDZ1) you have max 3TB of data to back up. Order an extra 4TB hard drive to act as the backup along with a USB HDD dock. Copy everything over to the 4TB drive.
When you are finished keep that extra 4TB drive around as a cold spare.
If you are flush buy a pair of 8TB drives, that way they are big enough to backup your new pool (assuming you are going RAIDZ2 this time).
I always have on hand enough storage to backup all my pools, I usually buy the drive size I intend to upgrade the pools to so that when I'm ready to upgrade I buy the remaining drives I need and then the new backup drives.
I do this because I prefer to spread my pool drive purchases over time - I hate it when all the drives wear out simultaneously.
Are you keeping the old pool?
If I can think of a viable use for the other pool (or, more accurately, the drives that were the other pool) I'd like to use it. The primary purpose of the machine is to be a Plex server. It's a media repository.
I'm not sure how smart Plex is about looking in multiple places.