The general recommendation is, if you don’t need any new feature that is required by the upgrade; there is no need to upgrade the pool.
This is because pool upgrades are irreversible and not backwards-compatible, potentially removing the ability for you to run older versions of TrueNAS should you ever have the need to revert back to older version for whatever reason (eg. stability or recovery).
Didn’t answer my question though…
That said, I hear you. Without having any idea what changed with the version bump; I imagined if there were any changes related to this relatively new feature (VDEV expansion), perhaps related to expansion performance (it’s really slow), then I want them…
Expansion is going to be slow as it has to calculate and move a lot of parity data into the new drive. You didn’t specify how much data you have, but depending on the size of the array, it could definitely take a while.
Re-read the Release Notes, as you should have read them a first time BEFORE updating TrueNAS…
Re-read the Release Notes, as you should have…
The only relevant release note was VDEV expansion! 
Everything else is incidental.
Expansion is going to be slow …
Oh yeah, I understand, and that’s fine. The question was; is it safe to update the pool version while it’s expanding? My guess is yes, but I didn’t want to tempt fate. I’m sure nobody would allow to release anything so brittle though.
