Those plugs are ever so slightly different, you can't mix the 2 up and get a proper connection. If you unplug the box, just make sure the car has powereddown for 5 mins (if you listen carefully you can still hear a slight electric buzz from under the bonnet for a few mins after you've locked up - you can definitely hear it click off). I usually pop the bonnet, lock the car and leave it 5 mins before disconnecting or reconnecting the box. The extra power is definitely epic, the car will feel so dull without it when you drive it in for service/warranty work. As long as you don't significantly change your driving style (just get up to speed faster), you shouldn't see your mpg appreciably down on stock.
