As far as I’m aware, the OS is stored separately than your data on the devices. You should not have any loss of your personal data. 

Have a talk with your family before you brick your CR-100s. I’m still catching hell on a regular basis over my decision to upgrade.

I’d have been a LOT better off leaving the spouse’s stuff on v8 and moving the stuff I wanted to upgrade to a new system/setup.

Yes, you make a good point and although my wife says she understands the situation (constantly replacing CR100 batteries with inferior Chinese Knock-offs which aren’t even available to ship to South Africa at the moment) and the advantage of being able to stream Apple Music, I know it will come back to bite me. I do have 2 iPad Minis which I was going to replace the CR100s with but tech is not her strong suit …!? 

Is there a fail safe guide on how to leave certain components on 8.4 and upgrade the rest? Will the Sonos App not want to automatically update everything around - I read warnings about visitors with the Sonos App on their iPhones inadvertently updating your system when it’s been successfully frozen!

Ideally I would leave her ZP100 and ZP120 on the old 8.4. Then update the 3 x ZP80/90s, 2 x Play 1 and the 2 new ZP120s to the latest and install the Sonos App on my iPhone and iPad. Would apps like SonoPad and SonoPhone (which I use all the time) ‘know’ which system they are intended for? And would the Sonos App also ‘know’ this?

This is going to have to be planned like a military operation to avoid the war that will start if I get it wrong!

Also that way her playlists are guaranteed to be safe!

Can a NAS drive serve 2 networks? Otherwise I’d have to buy a 2nd NAS drive for the music.

You don’t need to split the network. Create two separate Sonos households (systems) by factory resetting the units you want to upgrade, plus the controllers to go with them. Then simply set them up as a new household from scratch. If the household’s to be used in SonosNet mode it will require its own wired connection.

The new household will of course need setting up with NAS library, online accounts and so forth. Any Sonos playlists from the old household obviously won’t be carried over either.

So when I set up the 2nd system which will be allowed to update to the latest firmware, will the Sonos app give me the opportunity to create a new account with a different email address? And having done that, will the old system still on 8.4 continue to be safe from updates through the app?

You don’t need a new account/email address. The current one will supply the latest available software for the new household, assuming you haven’t blocked connections to the update servers. 

As for the 8.4 household remaining ‘safe’ it depends how you police access to it. It will require its own controllers and/or apps, separate from the new household.

Thanks Ratty, but once I start updating won’t Sonos automatically look for all my zoneplayers and deliver the death knell to my CR100’s?

No, because the controller is only talking to the players in its own household. When you set up a new household it creates as new, unique, HouseholdID. This ensures that only the players and controllers which are meant to be part of the household can talk to one another.

If you’re feeling excessively paranoid, power off all the ancient 8.4 kit while you set up the new household with the factory reset units/controllers.

Last question Ratty: Must the units I want to update be reset first? Then I add them one by one through the app and that way they belong to the household you are referring to?

Sorry, I didn’t read your reply properly - you do say “factory reset units/controllers”. Thanks for all the advice - I will go and get started!

If you’re nervous, just factory reset one player to start with. Reset a controller app and then set it up as a new system.

Once you’re feeling confident, reset the other applicable units and add them.

My wife has just asked again - any risk to her playlists? I presume they’re safe as long as I don’t reset her ZP120’s! Speaking of which, is there any way to backup the Sonos playlists? I have previously manually made iTunes playlists from screenshots of the Sonos playlists since they can be backed up, but its a lot of hard labour.

The playlists on the units in the old household will remain intact unless you deliberately mess with them. 

There’s no official way to export Sonos playlists, but you might be able to find a third party tool to do the job.

Job done. Playlists intact. Thanks to all who contributed suggestions, especially Ratty for the exact method and Stanley for making me rethink bricking the CR100’s!