(Assuming this is regarding a W210, 96+)
The cluster is re-programmable! You must have it programmed by the dealer otherwise TMU (True Mileage Unknown) will be displayed! And I think they would get the mileage from the old cluster, but if they couldn't for some reason, I'm assuming they'd go off service.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)