I have seen this.
The dealer can pull the Km out of the Km speedo then put in into the miles speedo. When you do this, it will convert all on its own.
I have done a few with no problems.
(Also, I have heard in rumor, not fact, that there are people who make the sticky face that can be applied and it is corrected for mileage)
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS