OK, tiny bit of history.
GM used to use a handheld device called a Tech1. Good little device, but was struggling to cope with the expanded range of systems on newer cars. When the Omega came out, it had so made onboard systems, the Tech1 needed its own dedicated software cartridge (as well as a multiplexer).
So, in 1997, GM moved over to Tech2 device, a much more capable piece of hardware.
So, in general, a GM Tech1 needed for pre 97 cars (including pre 97 Omegas), and a Tech2 for post 97 cars.
However, as Tunnie rightly pointed out, there were no significant differences between 96 and 97 Omegas, so often a Tech2 can read some systems on a pre 97 car, as long as that same ECU existed on post 97 cars, and a Tech1 can read some parts of a post 97 car, as long as ecu existed on older cars.
The cheapo Tech2 software may or may not reconise the pre 97 ECUs if they did not exist post 97, or if the software/firmware version the ECU is running didn't exist post 97. Knowing the flakey cheapo Tech2 software, it may not read it anyway...