The Port is the only device which is S1 compatible, so I would do that first.  

I presume you are currently using the S1 Controller App? If so, what is the reason you want to upgrade? 

Also, it may be unnecessary to keep using the BOOST. The Era 100 won’t use a SonosNet signal, it will need to be connected to your WiFi. Note I said unnecessary. You can continue to use it if you want, and your system will run in mixed mode, some devices on SonosNet, some on WiFi, without issue. 

Yes, that’s right.  The reasons are to be able to use Airplay 2 and to add to the system e.g. Roam.  At the moment if I want to use Airplay I use two UE Boom speakers.  I will retire these once the upgrade is in place. 

If you were a friend, my advice would be to hold fire. The current state of the new app is a world away from the stability you currently enjoy with S1. 

Keep an eye on the new app thread(s) and wait for things to settle down. 

Another option would be to buy say a Move 2 (they sound great) and just have that initially with the new app as a separate system. That may make it easier for you to judge when the time is right to start moving more stuff over. You should still be able to use the Move 2 with AirPlay and Bluetooth. 

In terms of the boost, I would avoid Sonosnet now and get your home wifi solid. The Era’s no longer use Sonosnet anyway and therefore you’ll need your WiFi to be stable for them to be stable too. 

As you have an S1 system, which I presume behaves at the moment, I would be tempted to move it off of Sonosnet by removing the boost and seeing if you can get them all switched over to just WiFi. At least then you have a stable comparison with a working Sonosnet vs WiFi. If you do that at the same time as going S2 right now, you’re going to struggle to know if the issues are new app or WiFi related. 

Finally on S2, if you have a Mac, there’s a ‘Discovery DNS-SD Browser’ app you can install which allows you to see what Bonjour/mDNS devices are on your network so you can verify that this is working. This is a key step in getting ready for S2 now, and having any chance of a decent experience. 

Hope that helps. 

Well that was fun.  I bought a Port and installed it into our Hi-Fi stack, resetting it so that I got a flashing green light was easy.  Then the fun began.  Using the current version of the Sonos app it just refused to downgrade so that I could use it with S1.  I read that this was not possible with the current version of the app but I persisted because wiring the Port into an existing stack up against a wall and with no direct access at the back is, well, difficult.  I got the flashing red light which I read meant that 30 minutes had elapsed and Port was still not connected.  I must have by now spend two to three times that time, resetting the Port several times.  A couple more attempts to downgrade and it worked!!  The Port is now working fine and providing an Airplay 2 access point.  I have learnt several things:-

1. The Quick Start Guide from Sonos is completely worthless.  For anything detailed you have to visit the web.  It looks very nice. 
2. Sonos advertisements which say that a device is S1 and S2 compatible don’t tell you about the downgrade procedure so I had expected it to work out of the box. 
3. Sonos advertisements don’t tell you that the downgrade procedure is problematic or does not work. 
4. You can make do with a single Airplay 2 access in a system by grouping devices together.  You only need to upgrade other devices if you anticipate playing different music on each one. 
5. It is not worth upgrading my other devices.  I am sticking with S1.  
6. I am not decommissioning my UE Booms. 

That’s it.  I wish I could remember the exact process I followed so that I could advise others.  I think getting beyond the red light is important.  We seemed to do the same thing again and again and suddenly it worked.  It was a war of attrition.  

For the upgrade I have 4 play1, playbar  do I get 15% upgrade credit for each pieces?

Check your Sonos account, not the forums, and you should be able to see what upgrade credits you have available. Usually it is one credit per device BUT you can only use one per purchase.

NOTE that “per purchase” that can be a single 100 or a full-on Arc Ultimate setup, anything you can add to your cart from the Store as a single item.

I would wait for a better sale if doing a big purchase. Sonos do 25% off and more occasionally.  Getting close to Black Friday if you live in NA. 

I think at this moment in time you would be mad to "upgade" a perfectly running S1 system personally.