Persians are not overly fond of Arabs and deep down they don't fit well with the Arab religion. I suspect there will be a return to nationalism and ethnic pride that will make them leave Islam en masse -- except for a few more Arabic types. Maybe they can't go back to Zoroasterianism but there's always Christianity or there could be a reformed Zoroasterianism which does allow converts or returnees. It seems like a number of Iranians are Bahis. Right after 9-11 it seemed every Persian I saw had on some kind Bahi t-shirt --- I think to make sure everyone knew they weren't Arabs or Muslims.
This is certainly to be encouraged.
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.