TrueNAS has very little ties to specific hardware. Mostly just network interface names in configuration, etc. CPUs definitely can be replaced. I am slightly worrying about mentioned 9.x version, it would be a long chain of updates.
i have swapped out motherboards and CPU's on existing FreeNAS and TrueNAS installations. Never had any issues except for the NIC configuration since i had a LAGG configured which didn't play well with my router.