My understanding is, if the One-SL is currently setup for S1, then it can remain on that older system, but once it’s upgraded to S2, it then cannot be downgraded back to S1 - there is no route back. All new One-SL speakers are S2 App only.

In your case, a ‘used’ One-SL speaker that’s operating on S1, should be capable of being added to an S1 system, otherwise any user ‘factory resetting’ their own device would not be able to add it back to their system. So I think if you can find a One-SL for sale  thats never been upgraded to S2, you should (hopefully) be okay, but if you still have concerns, then maybe check with Sonos Customer Support prior to purchase. 

I think I disagree, unfortunately….which is probably why you need to contact Sonos support to be sure. I think, in order to ‘add a device’ to your system, it checks the software running on it, and will update it to the minimum requirements as set by Sonos. So, when you add a One SL, it will get boosted to S2 automatically. 
 

This is likely why you should call Sonos to be certain. Either @Ken_Griffiths or I could be right. 

@MotherGoose,

Did you get a definitive answer on this issue? I’m intrigued to know that if a person has an existing S1 Sonos system only, that if they purchase a used (old) One-SL that has only ever been used on an S1 setup only and then try to add that to their existing S1 system, if it will install/work, or not?

Just looking here to perhaps broaden my own Sonos knowledge, as every day is a school day for me here, it seems?

@Ken_Griffiths - Thanks for piping back in here, I do indeed have an update. I’m sure some point in the future someone will Google for exactly the same thing!!

Contacted Sonos support and the answer is yes,  it CAN be done. So, to be clear…..

An existing S1 setup with an One SL. Purchase/obtain another old One SL that has NOT been upgraded to S2 use - so one that has only been used on an existing S1 then removed. Add that into my S1 environment and that would work.

It was reiterated repeatedly that once a player has been upgraded to S2 use then there is NO going back to S1 use.

I guess the hard bit is finding a One SL that hasn’t been upgraded. I’m UK based and quite a few on ebay.co.uk. Contacted a few sellers asking if the player for sale is used in the S1 or S2 app and the responses I get back tend to be “yeah, just search for the Sonos app in the app store”…….

Thanks again to @Ken_Griffiths @Airgetlam

*Edit*

On the off chance anyone in UK is selling a black One SL that has ONLY been used in S1 then leave a message in this thread.

Ah that’s what I thought would likely be the case too, as mentioned in my initial post. Thanks for clarifying the matter @MotherGoose . I was aware that once anyone upgraded an old One SL to S2 that there’s no going back to S1 aswell. 

I can see that the old One SL’s still running on S1, may go onto become a ‘collectors’ item …ha ha.

Note you can always buy a used Sonos One instead as that is apparently still downgradeable to S1 and it can be stereo paired, or used as HT surrounds, with a One SL too.

Note there are three versions of the One SL: S22, S38 and S43, the model # should be on the base. S43 is new and is S2-only.

@controlav - Ahhh, I did not know that!! That is very good info. - thank you.

That’ll certainly help narrow down a used S1 unit - thank you!

Like @Ken_Griffiths says…….everyday is a school day!!!