How to change os cm845-cm848

long story short i have a truck here that was flashed with a diabo box tuner from another truck. the truck it is off of is a 2003 the truck they tried tuning is a 2005. when put back to stock off the diablo it puts it back as a 2003 with the other trucks vin number and a cm845. i have gotten the vin changed back to the original but now i cant get it back to a cm848 os it says invalid checksum i need to get it switched to an 848 and i just dont have enough experience in this to figure out how i can do that what are my options here? TIA