Do you already have an S1 device? You need to already have one to downgrade an existing S2 device to an S1 device.

So for clarity.

I currently have 2 Play 1s and 3 Play 5s gen 1.  I bought 2nd hand, 2 Play 1s. which I now know are gen 2. The Sonos 1 app tells me to revert these to gen 1 and the help pages say I need to do this through the Sonos 2 app,, which I have on my phone now.  The Sonos 2 app can see the 'new' units and identifies there are other gen 1 units on the network.  

This is actually not very clear at all.  You cannot revert products between Gen 1/Gen 2. “Gen” refers to the specific product version that you own, not the software system installed.  S1/S2 are the two different software systems. 

Also, “hacking mobile unit 7” is the name of the wifi network your S2 system is setup on?  Are your S1 products on the same wifi network?

Sorry for my incorrect references.

The current system is an S1 with 6 units. This is via the 'Hacking Mobile Unit 7" (my son's humour).

I have 2 second user Play 1s that the previous owner used on his Sonas 2 set-up.

I have factory reset these so flashing green lights are showing.

My Sonas 1 system tells me to reset the new units to match S1 system.

The online help states Sonas 2 app will do this and I have installed this on my phone. This app 'sees' the 2 new units but fails to reset these and shows the prompt shown in the pic.

Your S1 app sees the other S1 Sonos with no problem?

Do you have the app permissions on the phone set the same for both S1 and S2 apps?

Hi @Vonb 

Welcome to the Sonos Community!

I recommend you get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.

I hope this helps.