i just rang the rta up about it, I can just put the new one in and have it going from 0km, they said the reason they record it on rego is so if they audit the workshop they can see how many km the car has done since the inspection, and since im not a dealer or anything i can just change it, said its best to keep the old one so when i sell the car i can give them the old speedo as well, hope this helps other people thinking about the same thing,
since im putting a new engine in ill keep the 180kph dash till after i get the blue slip done and keep a copy of the blue slip with the old cluster