@Anton

As you are still on 16.1.1 with both, app and firmware, imo it should work as expected to downgrade the play1s and the sub2. 
I recommend to turn off all other devices that you are going to keep on 16.1 firmware so there won’t be the risk of updating them by mistake. 
 

If the play1s and the sub will need to be updated first, you probably will need another controller device where you temporarily can install latest S2 app and start the downgrade from there. 
 

First of all, I would reset one of the play1s to test this. After resetting it and turning off all other devices please try to downgrade it via 16.1 app. 

@Schlumpf 

Thanks. I was hoping to perform the downgrade from my existing app without having to load the latest version onto another device as it does present the option in 16.1

I’d be wary of the latest code too. If you get it wrong the 16.1 controller won’t be able to do anything with a speaker on the very latest S2 firmware.

@Ian_S 

Thanks.

I really want to avoid updating any firmware beyond what’s there now (16.1.1) on devices I want to have on 16.1 S2. I’m plucking up courage (not the Sonos type ) to power everything down, reset a play:1 and try the downgrade in the 16.1 app and then add it into the S1 app as a new device.

Sonos - putting the fear into courage 

@Anton 

I cross my fingers… but as said, I think it should work from 16.1 app. 

I’ll give it a go sometime this week and post results

UPDATE

Success! I now have all 4 Play:1s and the Sub on S1 with the Beam, Roam & Five on S2 16.1. The S1 system is for music use only and my library can now be accessed by this system from an SMB1 share via my router. I’m aware of the security issues with SMB1 but it’s all contained locally within my home so I don’t really have an issue with it. My router only supports SMB1 and I don’t see why I should  have to replace it or buy a SMB2/3 NAS just to get Sonos to work.

I’ve also been able to add some internet radio services and the S2 16.1 devices are working as expected too, so I’m very happy.

In brief, this is what I ended up doing:

• Power off all speakers
• Factory reset a Play:1 and left it powered on
• tried to downgrade it using the 16.1 app but no success as the app said it required an update
• Loaded the newest app on a second android device
• Added the Play:1 to this system, which prompted it to update firmware
• Once complete, this allowed the newest app to downgrade to S1
• Once complete, powered off the Play:1
• Repeat for all remaining Play:1s & Sub
• Deleted new Sonos app
• Power on all downgraded speakers
• Installed S1 app on phone
• added all downgraded speakers to S1 app
• Power on remaining S2 16.1 speakers
• Check system, all working as desired

I intend to leave the S1 devices alone from now on and IF Sonos sort out the S2 app, I may replace the 16.1 app with the latest version, but it’s staying on 16.1 for now.

Thanks to @Schlumpf and @Ian_S for your input

@Anton 

Congrats and many thanks for the input as I wasn’t sure if the fw update and new app before updating are necessary. But now I know… they are.

Yeah, if you're on 16.1, you need to upgrade to downgrade

LOL… wisdom of the day!