Okay, here is the theft relearn procedure you can try. Follow it exact. You still may have an issue though because you cant edit the vin number of the bcm to input yours. It is locked on most cars. You can attempt if with the tech 2 in bcm setup to see if it will let you change it. Probably not. So if this theft relearn wont start the car, then you need a new bcm, not a used one. But hey give it a shot first. Here is the relearn.
30 Minute Learn Procedure
Important: The vehicle learns the password XXXXX the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
As long as you are not changing the lock cylinder it doesnt matter. The cylinder reads the key code, but the new bcm must relearn them also. Take, Care, thanks, Eric