Hello and thank you for your post. I am a master certified GM tech, with 15 years Saturn experience,and can help you to resolve your issue.
If you take a BCM from a used car and put it in your car, the miles will be wrong on the odometer and the car will not start.
If any options of the car are different such as type of a/c system, power windows, keyless entry etc... then the bcm will have to be reprogrammed to match your car.
either way the security system needs to be relearned so th ecar will start.
To do that, turn the key on, turn th ekey to crank and let the key return to run for 11 minutes.
turn the key off and repeat 2 more times
after the third time take the key out for 1 minute and the car should start.
if it doesn't then the bcm will need reprogramming but never will the mileage be correct. Mileage is not reprogramable.
if you need add'l help let me know.