@gogscurrie
With the Beam2 being the master device of your home theater setup the whole setup is atmos ready. The Beam does the decoding and delivers each channel information to the bonded sub and rearspeakers. So they for themselfes on their own don’t have to be atmos ready.
But the Era300 although are… the Era100 aren’t, but used as rearspeakers both Era models are optimized for atmos playback, because they are able to play separated atmos rear channels they get from the master device.
In cse of Era300 all rear channels are real… Era100 just play the back rear channel real and the back hights and back side channel just are vitualized.
Sonos One(SL) does no such separation but plays all of these atmos rear channels in combination.
So yes… imho also just a pair of Era100 as rears (with optimized positioning and room conditions) will be an atmos upgrade to a pair of One(SL). But probably just a small upgrade and I‘m not sure, if everybody will be convinced of the difference.
Just buy and try… 😉