I was told by GM that only a "virgin" odometer can be programmed by a Tech II programmer to a set mileage. Once an odometer sees it's first BCM signals to start recording mileage, it locks out the setting mode and can no longer be reset by a Tech II. Makes sense if you think about it - an dealership could roll back mileage on every used car on the lot.

There are a couple of specialty reprogramming shops that you can google. There are a couple certified by GM (probably some government regulation too) to be able to set a specific mileage in a non - virgin odometer. They are pricey though - over $200 bucks for the programming plus shipping. I decided to use the second DIC behind the 5 button, and swap as needed.