Forgive me if this has been asked and answered, but I couldn't find it anywhere. I have a turboed 01 GMC truck with a 12208322 OS and want to up grade to the custom 03 OS for obvious reasons. Do I need to load the 01 OS and appropriate .cal and .tun files first, and then load the 03 OS? Or can I just load the 03 OS and which .cal and .tun files should I use? Thanks