If you look through these forums you’ll see this theme repeated many times. 

It can’t be done, at least not automatically. 
 

You can go through the process of de-bonding them (and the Sub) from the Beam, then stereo-pair them as a new room and bond the Sub. Then, of course, reverse the process to get them back to your home theatre setup. But it’s slow and cumbersome. Plus, if you use the full Trueplay, that needs to be re-done each time too. 

Some folk use the settings to set the surrounds to Full rather than Ambient, and turn the volume bias to maximum to the surrounds for music play. Then you probably can’t hear the Beam, though of course the “stereo sound” is coming from behind you rather than the traditional in-front location. (But were you going to re-position them each reconfig per your original question?)

Thank you for that Nik. Wow, this is so disappointing. If I'd known that before I probably wouldn't have bought a Sonos system. I'll probably sacrifice the Home Theatre and keep the Era 100s for music only. 

Or - (more money! gasp!!)- buy another pair of Era100’s. Then set them up as a separate zone stereo pair. Save a lot of time bonding/unbonding etc - and no - I don’t work for Sonos sales department. 

Setting the surrounds to Full rather than Ambient, and turn the volume bias to maximum to the surrounds for music play is actually the best solution but I think Sonos should offer a better alternative for this problem. 

There is no alternative.  In surround configuration, the radios are reconfigured to a one-way dedicated 5 GHz connection from the soundbar to the surrounds.  This requires a series of handshakes to get right, which is why adding surrounds takes a bit of time.  The reverse happens when the surrounds are removed.  So therefore, a quick toggle is not possible.  They could place a hot button that does the add surrounds/remove surrounds at the touch of the button, but that is not going to speed up the actual process, and doesn’t save much from the system menu commands we have today.