As someone that has a unified S1 system with some S2 capable units, I have researched this subject and what you describe flies in the face of all I know.

One, units cannot move to S2, without first downloading the S2 app on the controller.

Two, there is no forced updating of the kind you describe that has ever happened so I doubt that it is happening.

Three, reverting from S2 to S1 is possible via a systematic process, for S1 capable units.

I would be happy - and alarmed - to be corrected by other members here on any of the above, though not so much on the last one because I will never move my play 1 units to S2.

I’d love it if you were right, but unfortunately that’s not what happened in my case and I have talked extensively with Sonos Support directly about this.

I have recently moved and have been slowly unpacking and re-adding items to my S1 system. For the last few weeks all I have had was my One SL and an old Play 3 working perfectly well on my S1 app, but this morning my S1 app told me all my speakers were compatible with S2, so download that instead (there was no option to stay with S1). I thought I might be able to solve this by unpacking my old legacy Connect. That got me into S1 again, along with the Play 3, but when trying to re-add the One SL the app told me it would only work with S2.

Support told me the update to the One SL that caused this happened last night (July 19), but a) I wasn’t home to have done any updates and b) I don’t have auto-updates turned on (something support confirmed with me).

They told me this was pushed to all SL units, but that seems like a pretty big deal, which is why I’m posting here to ask if anyone else had this happen.

Now I am not worried - I know already that come hell or high water, I should not download the S2 app as you did and then I can have no issues of the kind you are now having. I just keep ignoring the rare S2 baiting that happens via messages in the S1 app, and all remains good.

I believe there is a way you can get the SL to go back to S1 - google is your friend for finding that, or anyone else that may post the process here for you.

I never downloaded the S2 app. I too have been refusing!

Unfortunately, as confirmed by support, there is no way to reverse the S2 update to my One SL (otherwise I would have just done that and not posted here). Resetting the device has no effect and I’m told the update is irreversible.

Hopefully this is just an issue for me for some reason, but figured I would check here in case others had the same problem.

Please say what phrase to type into Google (or provide the link, if you dare), because

https://support.sonos.com/s/article/4786?language=en_US

(now) says: “ 1] One SL can only be added to S2 systems. One SLs that are already part of an S1 system will continue to work as they do today.”

which I don’t remember from several months back when I set up an S1 system including an old ZP with several OneSL’s.

You may be right - and that is why I had said I believe. I am happy to change that belief if something funny is going on with SL units as you describe. I don’t have any, and neither will I ever want to go back to S1 with the other Sonos units I do have because I will never leave S1 in the first place.

But how the OP SL units moved from S1 to S2 in the first place is still a mystery.

Yeah, the specific edge case not completely beaten to death and nailed down by @James L.  in the pretty-clear answer (if you read the whole thread)

https://en.community.sonos.com/what-to-get-228989/sonos-one-sl-s1-compatibility-6857860

is: If I bought 6 OneSL’s in December 2019 (good price!) and integrated 4 of them into my S1 system, and put 2 into the basement for use as spares when/if the old ones died -- because the spares are unregistered am I prevented from ever adding them to my S1 system? Should just sell them on now as NewInBox?   I.e, is the phrase in the dealer notice “or unregistered” absolute or does it depend on whether they were originally loaded with old (pre-May-2020) software or new (which while the feature was still available, were actually “downgraded” when added to an S1 system)?

Agreed how the OP’s OneSL moved from S1 to S2 is a mystery.  But I have seen plenty of “unexplained” upgrades in the old software when “someone” (who shall remain nameless) brought their new phone home and merely used the system, of course agreeing to update the boxes to match their new controller, aargh.  It’s a little harder with S1/S2 migration but not out of the question...

I would think that not being able to go to S2 without first downloading the S2 app should be a good enough barrier to some amount of careless behaviour. Not all of it, I agree:-).

@p_wats - I’ve sent you a PM 

@tracker - Those unregistered One SLs should be able to be set up on S1, so long as they’ve never been set up on any S2 system previously. Since they’re from December 2019 it’s safe to say that they won’t contain S2 firmware.

As for the question around dealer notices - I can’t confirm for certain, but I don’t believe “unregistered” provides any guarantee on what kind of firmware a device is running. It seems to just be a notifier as to whether an item is A-Stock (Factory New) or B-Stock (Factory Refurbished).

Commenting here for posterity, in case anyone stumbles across this thread. After support calls and private chats we seem to be at an end of discover-able details on what happened with my One SL.

Sounds like it was auto-updated to S2 after I reset the device to add it to a new S1 system at my new house, even though tech support confirmed I had auto-updates off and I have never used/installed the S2 app in any way---always only S1.

As per @James L.:

It appears that the One SL was reset, set up via an S2 app but the update process was never completed (I can't confirm an exact reason why). It was still running S1 firmware until the automatic update process kicked in on the 19th

There doesn’t seem to be an explanation of how it could have been “set up via an S2 app” even though I have never downloaded the S2 app.

Unfortunately, this has also been proven to be non-reversible, so I’m stuck with two systems now.

A Sonos sales person I spoke to on the phone even said “sounds like someone was trying to force you to upgrade”.

Word of warning to anyone relying on a mixed S1 system, apparently it is possible for your S2-capable units to upgrade without you ever touching the S2 app and there may not be much Sonos support/sales can tell you other than “use two apps now”.