That is my understanding as well, BUT the "New" Sonos product you add has to be S1/S2 compatible and needs to be added to the existing Sonos system as an S1 device.

In my experience, AMP/Ports/S2 compatible Connects/Connect Amps, will add to an S1 system automatically if they are new out of the box, or have not been activated on S2. If the item has been activated on S2, it will need to be defaulted and downgraded.

This link might be helpful.

[Link: support.sonos.com]