Yes, it is for all Z40 models.
I will post a few messages from a user who said there were issues with video calls after update to 1.6.
We fixed it in the end, but it was extremely painful and we still don’t know if it is a bug, or if he was just unlucky…
I would not update unless you have a problem.
You will not be able to downgrade if you take the OTA.
FYI, v1.6 was only made to work with newer Pixart chip which is used in the latest production watches. Apart from that, it makes no difference to older watches purchased over 6 months ago.