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I don't see why not. Once you upgrade it, it IS a 75D. Says so on the 'About' screen and in the My Tesla account, etc. And the badge if you had them switch it.
I suppose so. The idea of a car changing its designation after purchase is something that obviously the motor vehicle departments probably didn't consider would be a possibility. If one was prepared to spend the time, effort and paperwork (the bureaucracy!) and get documentation of the update from Tesla, you could probably have the title changed, too. Not that it would be easy or seamless. But may be possible - but probably not worth the effort.
I'd advertise it as a 75D. Inside the ad though, best to say somewhere that it started life as a 60D.