There's an option in the MMI to specify units. Haven't tried this yet, but I'd imagine it would change the speedo appropriately?
Can't remember exactly how to get at it - think it's under the Info button, then maybe setup, or one of the corner menu items under Info.
I drive a RHD model in Germany - I changed the digital display to km/h when I picked the car up last October. I can't remember exactly how, but I think it was through the MMI. It was certainly only a matter of a minute or so to do it.