Best answer by John B
Having said that ... it always feels slightly safer to me to do the following, though probably OTT.
1. Wire one of existing units to the router temporarily.
2. Check after a minute or two that everything is connected by SonosNet - in About My Sonos System the speakers should all have WM:0 next to them not WM:1.
3. Add the BOOST UNWIRED (you have an option for this when you add it)
4. When the Boost has been added to system, just swap it with the temporarily wired device.
5. Give it a minute or two to settle down.
Please let us know how it goes.
(If you already have a wired device, just follow steps 3-5 above.)