Great work! Jobs like that can really go on your nerves but they have to get done. I am facing a similar horror trip with a Saab that someone hacked the wiring because they didn't have a Tech2 and started cutting wires trying get things to work that had been turned off in the system. Its going to be a huge PITA.
On these cars I use a Tech2 to change the VIN in the ECM to install parts that have been previously used in another vehicle, you know the divorce and marry parts to this ecm by VIN or they are not recognized thing, would the Tech2 change the VIN in this truck also? Do these US GM vehicles require security access to change some settings also?
Nope....Got to use SPS. I don't use my Tech2 as a passthrough (J2534) much anymore either after "bricking" a PCM with it. So I bought a MDI for all GM SPS programming. Mine being an original Hewlett Packard made one might have something to do with it?
Another oddity is.....You can't unlock radios on GMT800/GMT900 trucks with a Tech2, got to use SPS. But you can on all the passenger cars

IIRC....Isn't Saab/Opel a whole other system & uses it's own proprietary card software?
I can change the VIN with HP-Tuners, Though I didn't on this one because GMT800's can easily be stolen with a screwdriver anyway.