I think if you try and modify your new clock, you may end up making it look you've clocked it for the worng reason (ie reduced the mileage) - Which means you look dishonest, when you were actually trying to do the right thing!!
However, if you keep your old dial, you can show what the mileage was when you changed, and I would take a couple of photographs showing the install of the new clock. This confirms the mileage of the new dial when it went in.
It will then be very easy to identify what mileage the car has done, you get your new dial without running the risk of fcuking it up, and you have proof of the mileage covered by the old dial.