The Zoroastrians are in a bind. Like the article said, if they openly proselytize or are considered to be too aggressive, especially in the face of Khatami's relative laxity, then they risk angering the government and bringing down its wrath. Under such a cloud of tension and suspicion, they don't have much choice but to say that they will not accept converts. So for now there has to be this awkward situation where people can only convert and educate themselves in sercret. But in a future where there is freedom to chose religion in Iran, if millions want to go back to their national religion, it would be impossible to prevent them from do so.
I think many will go back --- there is a part of their original culture still in them --- you never see Persians act as weird as the Arabs act. Even Persians who are strict followers of Islam seem to know quite a bit about Zoroasterianism and seem to almost wish they were still that. Deep down I think most of them prefer the religious tolerance they were allowed to have back then, the more openness and freedom, and when the women could be beautiful instead of acting ashamed in hideous brown rags.