You are correct. You have to swap out the head unit if you already have a X4D system and want to upgrade to surface speed. I checked again with Trevor and he told me his first answer to me was incorrect because he thought he was replying to someone with a X4 unit. I keep wondering though if the new unit's surface speed will be as accurate as my GPS SOG? Is it worth a 300 dollar plus swap just for the surface speed? Isn't there also a app you can download on your smart phone with the new unit?