Same exact issue a month ago.  Sonos eliminated the downgrade feature for some reason this spring/summer.  I immediately thought they were trying to force the community to pay up and move forward.  Called Sonos and their canned response was it will be released in a future update.  Still waiting….unclear if this is even prioritized as I haven’t seen it listed in their release calendar.

@Hugothom , that ‘feature’ was reintroduced a couple of updates ago, which is why the OP is concerned, it isn’t working for them. If you don’t see it in your controller, you may need to update your controller. 

OK so I managed to solve this.  In case it helps anyone else I’ve outlined what the issue was below.

I decided to try and add the speaker to a new system under the S2 app.  It initially wouldn’t add as needed an update to the software.  The first update attempt failed but after resetting and restarting phone with the Beam wired to an ethernet socket the update went through.  Once the software was fully up to date I then proceeded to attempt downgrading to S1 and this time the downgrade went ahead which took a few minutes.  This was in stark difference to the previous attempts were the app reported it was downgraded but had clearly done nothing.  Once the downgrade was complete it was easily added to the S1 app and all is now working perfectly.

For info this Beam Gen 1 was brand new but had been left boxed since purchase in 2020, I believe.  The software was therefore very old and I suspect this caused the problem with the Beam unable to downgrade without first upgrading!  To clarify in case someone mentions it was probably the ethernet connection which solved it I had already tried with ethernet previously and it still failed.  It was only after updating the software with the S2 app that I was then able to downgrade to S1 software.

Hope that helps anyone else who experiences this problem although I suspect few have devices running software that old!!