The dealer that installs it will have to reset the mileage on the new part to correspond to your current mileage on the old part.
AND...how many miles do you have on your car? If you write a nice letter or email to BMW customer relations, they might replace it for you for free, and you just pay for labor. That's what I did for mine, I had about 80k miles then, and the car was just over 3 yrs old (out of warranty becoz' of mileage). They did it for me out of goodwill, and also coz' it was a common manufacturer's flaw in the instrument cluster. I paid $78 for labor and remained a happy and loyal customer!
Here's a contact person but I'm not sure if she still works there,
She will respond promptly, and refer you to a local dealer if she approves of it.
Leonora.Vlaovich@bmwna.com
Good Luck!