Sonos is on record saying that updates to S1 will not degrade any existing functionality. What you don’t want to do is to update S2 compatible units to S2, leaving your S1 units behind. While that is unlikely to happen via automatic update selection in the S1 app, best play it safe by opting out of these and selecting to update once in a while from within the S1 app. There is nothing to lose by the delays this may cause in implementing updates to S1; these are just security updates and the delay isn't going to cause any great harm.

There is a Sonos support page here that details any major changes in each S1 Sonos update:

https://support.sonos.com/s/article/4941

Minor/interim updates and ‘bug fixes’ are not added to that list and in that case there is unlikely any changes to the functionality of the system.

Thanks Kumar for your response. I had already opted out of the automatic updates and will continue to do so although normally I do update automatically to ensure my software is kept updated.

Is there a technical difference between accepting an automatic update and manually updating later unless some details of the update are published?

I suppose in any case if you don't accept an upgrade your system is frozen in perpetuity.

There is no risk that I know of in accepting automatic updates within the S1 app; moving the system to S2 needs the S2 app to be downloaded first, and if you do not do that, the system units should not move to S2. Similarly, there is no real risk in doing manual updates later as long as you invoke these from within the S1 app.

There are people that have stopped accepting upgrades for a few years now, from even before the S1/S2 thing and are quite content with system performance; they see no need for any more features in exchange for possibly obsoleting functioning hardware.

None whatsoever. I’ve never let my systems automatically update.

The automatic update switch in the controller only applies to the Sonos system firmware. If you want to remain frozen in time you’d also have to stop your mobiles from auto-updating their controller apps. 

Good point; but I don't supposed that any mobile will move the controller from S1 to S2 via any auto update feature. Yes, the units will then have to be updated but all will still remain in S1.

I believe that Sonos has made a distinction between updates, which in the case of S1 will always just be bug fixes and security patches, and upgrading from S1 to S2.  I don’t believe there is any possibility of being upgraded from S1 to S2 by an update, whether automatic or otherwise.

Edit: having checked, there are in fact places where Sonos documents talk of updating from S1 to S2.  I am still confident that this won’t happen as a result of setting the automatic updates flag in the app, as @Kumar has already correctly stated.