Hi Paul,
That's really not a good idea at all - while downgrading the firmware may allow you to output your captures in .jpg format, you would also lose the benefit of the many, many improvements that have been implemented over the years. Using a historical version of firmware could have very unpredictable results, and we we would be severely limited in the support that we could offer you if doing so.
Please see my response in your other post here, it really is pretty straightforward to extract source .jpgs using Immersive Studio, even if you are using MacOS.