While I dislike your question[0], it is a really good question! Because you're correct that a reasonable number of Zorastrians may not consider converts Zorastrian. Frankly, I don't know, and I don't know what a quorum of Zorastrians would say (from my experience, the majority would consider them Zorastrian).

What I will say is, I do think this question/your perspective is archaic (sorry for the harsh words, I have biases too :P). And I'll try to explain in the most HN way possible (i.e. the reasoning which convinces me the most, about most things).

Personally, I don't know these people, it is unlikely you do either. By any reasonable estimate no person knows any other person. All we can know are the inputs and the outputs provided to the black box which is "a person". As such, I subscribe to the idea that, the only real test which can be applied is one suggested by Turing[1].

If these people, including Julia Roberts, act the part they claim to be, so well that in a "blind" assessment they act so flawlessly that the observer cannot tell otherwise, then until proven otherwise, they must be Zorastrian or Hindu.

P.S. I know I need to work on being more succinct :)

[0] Respectfully, - Are there any real Scotsmen? - If there is such a thing, what makes a real Zorastrian? - Which person/people alive have the right to judge?

[1] If you are aware of a better assessment technique, sincerely, I would be overjoyed to hear it!