I'm pretty sure you can't do this. Not only will you have to replace your OnStar computer, but the trucks main computer as well. You will need to get it programmed most likely and install some new wiring and what not. Most likely you will run into many issues. Either upgrade to the 09 models or find an aftermarket bluetooth company.
I contacted a few dealers and they all told me there is no Retrofit kit for the Bluetooth. If you look at the 09 and up models it is integrated into the steering wheel as well. There are a few nice kits out there, but like you, I wanted a dealer item. Not worth me upgrading to an 09. I can do without.