> It might be nice actually to have orig_nbformat keep track of multiple conversions, too. Currently (I think?) it will be overwritten between conversions, so if you go 2->3 and then 3->4, you'll lose the fact that it was originally 2.

I'm not sure what the used would be. I suppose having `oring_nbformat` help to debug is there are some bugs in the conversion process to determine that there was a conversion process. I'm not even sure we should really keep it is the notebook is loaded and re-saved. Indeed the field might follow a notebook for years, even when the content does not have anything to do with it anymore. This is also one of the reason I would push that into MD.