I would perhaps keep things really simple and power off the devices currently in your original S1 system and that will ensure you do not interfere with that setup.
Install the S2 App (or reset it) and then setup the Roam from its factory state, using the S2 App. Then install any music services, like it’s a ‘new’ system and when done, power on your S1 system. Use the S1 App as before for your older original devices and use the S2 App for your Roam.
If you do have any speakers that are both S1/S2 compatible you can choose to use those with either Sonos App (that’s a matter for you to decide later). The compatibility lists for S1 and S2 Apps are shown here in this link:
https://support.sonos.com/en-us/article/sonos-s2-compatibility
Let me know if you need any further information.