David, I would be having a fit with GM. Isn't there another GM/GMC/Pontiac/etc dealer in the area? If so, take it to them and they should be able to handle it. You must be in Europe... Or Africa.
I'm getting pissed thinking about it....
The issue is not whether you can make the adjustment yourself, but should you. And the consequences that go with your decision. Put the ball in GM's court.