Don’t over think it. It is all in the app. Remove the current surrounds, connect the new speakers the pair them with the Arc. You may need to update the 300s before you can pair them.

Be aware that I attempted to do this very thing and it’s been a huge headache. In fact, I can’t get it to work after working with support for over two hours. My system— consisting of the Arc, Sub, and two Era 300s—is entirely hardwired so if your system is on WiFi then your mileage may vary.  But I will say this: I’m close to abandoning Sonos altogether because after years of enjoyment, the brand’s control system is no longer reliable. 

The Era300’s are the surrounds for the Arc? And you have the ethernet dongles for the Era300’s to allow ethernet connection? 

Yes, although they’re not the Sonos branded adapters. I already had two adapters and used those. The underlying technology of USB-C to Ethernet should be brand agnostic. 

As above, in the app first remove the One SL’s as surrounds… 

https://support.sonos.com/en-gb/article/remove-surround-speakers

There’s no way to delete base speaker from the Sonos app, so I would just perform a factory reset on them. They will disappear from the app as it will no longer find them. They will then be safe to move on. If you look on your account on Sonos.com, they’ll still be there for a while but will eventually disappear. 

For the new Era 300’s when you attempt to add them into Sonos on initial plugin, the app will force a system update on them (do it one at a time) to match the levels of your current system. Then when you can see them both in the app, you should be able to go back into your Arc settings and add surrounds. 

I make no guarantee that you won’t have to perform some kind of ritual sacrifice to the Sonos Gods to get all that to work smoothly as it should …. 

That may normally be the case, but…

When used as surrounds, the speakers *only* communicate with the soundbar using a special ‘bonded’ connection over the 5GHz WiFi interface. This is proprietary Sonos tech. Same for the Sub. 

So… my suggestion would be to unplug the ethernet adapter from the Era’s and the Sub, and that WiFi is *on* for all three speakers in the app. On the Arc, if you want to keep that wired, fine, but WiFi must also still be on else the special 5GHz connection used to talk to the Sub+Surrounds won’t work, which means they won’t work as you want. 

Be aware that by plugging in the Arc, there is no way to stop it creating Sonosnet, which has other implications if you have any more Sonos speakers, and the Arc will be using 2.4GHz for that. If you unplug the Arc from ethernet, it won’t create Sonosnet, but will still only use the 2.4GHz band for connecting to your WiFi as it uses the 5GHz band exclusively for the special connection to subs/surrounds. I believe this is true for all Sonos sound bars, see below:

https://support.sonos.com/en-gb/article/supported-wifi-modes-and-security-standards-for-sonos-products

You’ll notice all the soundbars are only listed under products that can connect to 2.4GHz networks. 

EDIT: Going to back track a bit here. I would do the above first to see if that gets you to a working system. Then, if these two articles are correct…

https://support.sonos.com/en-gb/article/recommended-settings-for-using-sonos-with-ethernet-networks

https://support.sonos.com/en-gb/article/surround-sound-guidelines-and-limitations

It seems to imply you can then switch to ethernet once setup. I would not be surprised if initial setup over ethernet was not a smooth process. I would also not be surprised if Sonos say non-Sonos adapters won’t work. 

Thanks for your input guys.

I'm using wireless for my setup and I'll get them from a store where I can easily return them with no questions asked. So should I still have any issues I'll, they will be going back.

They should just disappear. Whenever I’ve factory reset any Sonos products for selling, they have never reappeared.

The Era 300s will set up as surrounds fine, just as simply as the Ones did.

The Era 300 is an awesome choice you want be disappointed. Just disconnect the Ones and connect the 300 should very easy.I have a dedicated Theater room they sound great.

I’ve struggled, recently with this exact same swap. I moved and still had the pair SL100s. They just wouldn’t connect until I did the wired option below (this was in July 2024, using TMobile internet)

The 300s, swapped in, were a little bit easier. I’ve never found the set up to be flawless and have always had to jiggle with it over 4 years and - across a few IP providers and different routers. I’m using IOS for set up.
 

Don’t ever call Sonos tech support. You will get better advice and protocols here. SonosSupport is a complete waste of time.

In simplest terms:

1. Have a 2.4 network 
2. Have a 5 network
3. Factory reset all components including the Arc

Now two ways to go from here

Again, I’m on TMobile internet. Their new modemRouter allows for separate networks, but not much else in terms of router control.

1. Wireless works as expected, you can experiment with connecting to both 2.4 and 5 networks - for me the 5g network worked fine for Arc300sAndSub 
   1. If it does not work as expected...
2. Wire the Arc, add the 300s and Sub
   1. After the wired connection is made, I found I could disconnect the wire and Sonos tech then established the wireless set up, as expected.

There is a real mix of good advice and bad advice on this thread, l Here are a few facts relevant to the OP's question. 

1. Swapping out one set of surrounds for another is a straightforward exercise.  Add the new speakers to your system, giving them unique names. I suggest Laurel and Hardy. Remove surrounds. Add surrounds.  Done.

2. Factory reset the original surrounds before you sell them. Don't reset anything else.

3. Sonos HT system are designed to use 5GHz direct routing for communication between soundbar, Sub and surrounds. Don't wire Sub or surrounds.