Yes it will just start searching for a valid sonar return but this will not be a problem. When you install an SD or map card the unit is programmed to read the information on it. If it’s a map card it will start displaying the map data for the area where you are (or where you have the cursor). If it’s an SD card it will start either loading Nav Data if the memory card has it or it will enter into the programming mode if it sees a software update file.

You can do this without the unit being on the boat but you would have to make sure and force the unit into the Normal operating mode just to make sure that the menus get reset to the factory defaults, else it will automatically go into the Simulator mode as Jaxsprat stated. The unit can make a software update from any mode but the menus can only be truly reset from the Normal (or forced Simulator mode). Since most boats have a transducer, power cable and battery source that can power the unit through the update process it makes sense to do your software updates there.