I have been following this thread because I have a similar 2019 Sierra and I am also trying to avoid that hefty dealer diagnostic fee just to check for a software patch. It is frustrating that there is no simple menu in the infotainment to see the actual transmission control module version.
From what I have gathered there was a fairly recent service bulletin in late 2024 and early 2025 regarding shift quality and shudder issues for the 8 speed units. Some owners have mentioned that if you have a high quality OBD2 scanner you might be able to read the current calibration ID but even then you would need to cross reference it with the latest GM technical service bulletins to know if it is truly the newest one. Since your mechanic mentioned a very recent update it might be related to the newer calibrations designed to address the hard 1-2 shift or the hunting issues when the cylinder deactivation kicks in.
