ive done the same to my old clocks. fried the display board when trying to change the led,s . i got some new clocks that had 166k on lol mine at the time was 107k. i just got my local garage were all my work and mot,s gets done to stamp the service book with the correct millage of the car and the new millage displayed buy new speedo saying speedo change all stamped dated and signed. as i looked into millage correction, and so i was told they cant change the gen 7 celica, toyota have made them anti-tamper proof, and was told this buy 3 different companys. so your best bet is to get some sort of reciept or service book stamped stateting what has been done with correct millage and new millage.