By definition of surround speakers, they’re stereo. The surround information from the right will come from the right speaker, the surround information from the left will come from the left speaker.

This is why surround speakers are required to be a pair of speakers. 

If I replace my Sonos one speakers with era 100 would it make a major difference if used as rear speakers?

Major, unlikely, IMHO, but I don’t have your ears. I’d suggest listening at a local retailer to determine if you can hear a difference. Especially for surround data, it feels like if you already own a pair of Ones, upgrading wouldn’t be a big change.

I’d probably feel a little bit differently about the upgrade if they weren’t surround speakers, but would still recommend you making the determination on your own. 

I’m in the same camp, surround speakers just don’t carry enough audio information that they need to be great. I have both Play 1 and One SL surrounds and they do the job adequately. 

If I had a pair of Ones as front speakers with no Sub I might consider going to the 100s if I could re-purpose the Ones in a different room but I don’t think the difference would be that great.

A single One where the dual tweeters of the 100 would add stereo imaging would likely get replaced. BUT note just how limited that stereo image is and make sure you’d be able to hear it.