Hi

I don’t have a split system as my devices are on S2. The articles below should answer your questions.

Setup separate S1 and S2 systems

https://support.sonos.com/en-us/article/set-up-separate-s1-and-s2-sonos-systems

Known limitations with separate S1 and S2 systems 
https://support.sonos.com/en-us/article/known-limitations-with-separate-s1-and-s2-sonos-systems

I would just power off your S1 system devices, that’s all of them. Then install the S2 App and create a ‘new’ Sonos system with your S2 only device. That will have an entirely different Household ID which you can use under your existing Sonos account credentials or even use a new Sonos account - it’s a matter for you. In my own case I used the same Sonos account.

The main pitfalls are … You have to install all your music services again to the S2 App and perhaps re-create any Sonos playlists you need etc. Also voice assistants, like Amazon Alexa, or Google can only each be used under one Sonos HH ID at any one time. Otherwise you likely won’t see any other issues.

I’ve never had an issue with my own S1/S2 setup that I still run side-by-side on my network with both Apps on my iOS/Android/PC controller devices.

Obviously once you have the S2 only device ‘up and running’ in the S2 App, you can power ‘on’ all your S1 devices and ne’er the twain shall meet, as they have different Sonos HH ID’s. 

If you later decide to move an S1/S2 compatible device from your S1 system over to S2, you will need to factory reset it and then add it to the S2 system using ‘Add Product’ in the S2 App. 

Thanks - I'd read the online articles and they all seemed to talk about only S2-incompatible devices remaining on the S1 network, leaving the fate of the ‘either-compatible’ devices somewhat ambiguous. Sounds like Ken’s setup is exactly what I’m looking to do so will take the plunge. Cheers

Recent talk seems to suggest that S1 devices that are compatible with S2 today are best left on S1 so as to not risk falling foul of S2 memory requirements in future.

I can not confirm that. Especially the newer devices that support both os eg. the Play5 Gen2 or the Move1 work quite good on S2. Even the Play1 does…

And if with future updates such devices come to their limits, as now on S2 Play3 or Connect Gen2 do, you still can downgrade them to S1 anytime. 
I‘m just happy to use as many devices on S2 as possible. 

It’s a bit dizzying, but if you want to transfer SONOS Playlists and music service registrations to the S2 unit, power down all but one S1/S2 compatible unit and upgrade it to S2. Now you can add the new S2 only unit to the upgraded system. Next, power down the new S2 only unit and downgrade the unit that will return to the S1 system.

Note in this case you do end up with the same Sonos Household ID for both S1 and S2 systems and in that instance, the system will ‘nag’ the user to update their S1/S2 compatible products in their S1 App and put them onto S2 - if a new S2 system is created, it will have an entirely different HH ID and they will stay independent of each other going forward. In fact I think I saw a post from @controlav recently about being ‘nagged’ by the S1 App and he actually has written an App to export playlists from S1 to S2 (and vice versa), but it’s certainly best to keep the HH ID’s separate for each system IMHO.

To each his own, depending also on what value is seen for the S2 app over S1. If/when older devices come to their limits under S2 iterations in future, it won’t be obvious at first that any disruptions are due to that reason. First you will work the “network is an issue” problem, that being the most common cause for units not working properly, and only when that fails after some attempts will you realise - if savvy enough - that the problem is the limit being hit and the solution is a downgrade to S1.