The only thing with doing at the dealers is the mileage will not be changed,
What ever mileage is on the clocks you get will be whats shown.
My dad recently had his swapped as the red bit in the middle faded,
Dealers told him they where unable to change the mileage on used clocks as it was illegal to do so.
They charged him £50 to code them.
We have since put the mileage back to what it was on the original clocks.