Yes.  A Gen 1 One can be stereo paired with a Gen 2 One.

I’m not sure quite what you mean by “a Sonos One Gen 1 that I upgraded to Gen 2”  There is no firmware or hardware upgrade available to achieve such a step. 

You can stereo-pair a One gen 1 with a One gen2.

You can’t pair a Play:1 with a One. 

 “a Sonos One Gen 1 that I upgraded to Gen 2”  =  I bought a Port to connect a audio technica turntable.  Sonos discontinued the ability to downgrade the Port to Gen 1.  My Sonos One was Gen 1.  So I went through the process of upgrading the Sonos One to Gen 2.  I can not use the Sonos One with the Port and listen to LPs.    I want to buy another Sonos One.  My choices are to buy a new Sonos One Gen 2 or buy pre-own Sonos One Gen 1 and go thru the upgrade process again.  I would rather buy Sonos One Gen 2 and want to make sure the two will be compatible to achieve a Stereo sound from the turntable.  

Sorry - I still don’t get it. Do you mean running on the S1 and S2 versions of the app? I still stand by my first post - that you can’t upgrade/ downgrade the memory or electronics changes in a Gen 1 and a Gen 2 version of the One. 
 

If you’re asking “what speakers are compatible with the different app versions” the details are here:

https://support.sonos.com/en-us/article/sonos-app-version-compatibility
 

Assuming you mean the S1 and S2 apps by Gen 1 and Gen 2… then the ‘downgrade’ tool is back on the very latest app and you *should* be able to downgrade the Port to S1.. And your One speaker. 

As the One is now discontinued, and a quick look here shows none ‘new’ available, you may well have to buy 2nd hand. Avoid One SL’s as although you can pair with a One for stereo, you can’t downgrade a One SL to S1, you needed to have bought a One SL when it came with S1 code to keep running it on S1. That must have been a very small window… and an even smaller number available 2nd hand that aren’t upgraded. 

If you don’t want to go back so S1 (Gen 1) then you could get a One SL, again second hand. The SL has no microphone which may be better if you use it, as you don’t really need two mic’s so close together in a stereo pair. Second hand I doubt there’s a big difference in price… 

All One’s and One SL’s will be compatible with your existing speaker once upgraded by the app to matching firmware. 

Thought I’d post this as a separate reply as editing the above could get messy!!

There were Gen 1 and Gen 2 versions of the One speaker, I think the main addition to the Gen 2 was bluetooth LE to aid initial setup. 

You *can* pair a One Gen1 with a One Gen 2 or One SL for a stereo pair. 

I suspect the upgrade you did was S1 to S2 firmware, which with the new app may well have been painful. *IF* you want to go back to the S1 app and firmware, this can be done with the very latest app, and your existing One Gen 1 and Port should be compatible. As I said above, avoid the One SL if you want to use the S1 app/firmware. 

Unless you have a local/online supplier with stock, The One (Gen 2) or One SL are only available used now.