Ask another dealer? I had an E60 in Germany and went from Radio Professional to Professional Nav. The way it worked was I first took the car to BMW to update to the latest software*, then retrofitted the sat nav overnight and took the car to BMW the following day to program the new unit with the specs of the car. I wonder why this wouldn't work for the 3 series (obviously I'm extrapolating here).
*It was a necessary step to first update the car to the latest software _before_ the retrofit coding.