Not exactly sure I’m following your questions, but if you setup  a Sonos room  as Arc +2 Era 100s, and simply unplugged your Era speakers when you didn’t want them for whatever reason, the room would still play.  Sonos amp would tell you that the speakers are missing and the Arc would continue playing and just not send any audio to your none existant speakers.  It would not send that audio to the side firiing speakers like it would if you had setup without rear speakers….if that makes sense. As a result, the Arc alone may sound a bit hollow at times.

The other factor is that you would want to be careful to place back into the original left and right positions.  If you trueplay tuned, I’d take even more care to place them in the exact original spots.

What you can’t do, and it sounds like you’re aware, is use the Era 100s as separate room from your Arc when you don’t want them as surrounds, without doing a setup/ reconfiguration every time you switch.

Thanks, you got the point perfectly and this is the reply I was looking for. 

Would the Arc not sound differently when it is tuned to work with surrounds that are not present?