Do smart curtain rods work with existing curtains?

It depends on the format. Retrofit robots like the SwitchBot Curtain 3 clip onto a standard rod or track and pull existing curtains, so they work with what you have. Full motorized track systems (Graywind, Quoya, Remac, Osprey) replace your rod entirely, which means you'll likely need to re-hem or re-hang the curtains onto the new track.