1) Do you need both versions? Why mod your primary, first device? How are you going to conclude It moar better
2) Wont happen. By doing something like that DJI would break the API-compatibility and cause lots of upgrade/downgrade troubles for 'legal' product. You don't change firmware unless theres important technical malfunction
3) You can configure a fake login, it's in the documentation ? Why worry?
In lieu of my advice, theres lots of reports of people using both versions side by side, should be no problem.
Oh, just make sure to disable any GSM data and Bluetooth before flying.