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Since July 2021, any ECU with a modified tune will fail a smog test in California; however, if the original ECU code is re-flashed, it's supposed to be okay and should pass a smog test.
My question is: Will reinstalling the original tune put the ECU be enough to pass a CA smog test, or is the ECU permanently altered once the tune is changed, even if the original tune is re-installed?
I doubt anybody except CARB and maybe the vehicle manufacturers know a definitive answer to your question. However, it is my understanding that CARB checks for the presence of OE calibrations at the time of inspection by comparing codes generated by the calibration in your vehicle versus codes the manufacturer's calibration generated. If they match, its assumed to be OE (and compliant). If they don't match, well, then it's assumed they aren't OE.
I know with later GM stuff has a "history" record of software "changes". I personally believe that the software needs to be stock and monitors run with no codes and it is good to go. Previous software could likely be removed if needed by using a replace module and TechLine Connect. However legal technicality is anything that will modify emissions. That could be broadly termed as you repairing a bad EGR cooler. During the time it is replaced it had to be removed. Did you modify the emissions? Well yes but it was during a repair ;) Similar to De minimis in HVAC. Long story short as long as it is "STOCK" it is good to go.