Have seen various discussions about replacing a Sonos speaker with a new or different one, but the old one still shows in the app as “not connected”.  I could not find a way to manually make this “not connected” disappear from the app. Tried reconnecting and removing again but no luck.  However, by simply waiting 72 hours or so, the “not connected” stopped appearing and there was no reference to the old speaker in the app.  (This was replacing a Gen 3 with a Gen 4 sub, but should apply to any other speaker.)