IIRC, Islamization in the Persian lands (including Khorasan and Transoxania) only really got started in the 9th century, and that due to the heavy infusion of Persians into the ruling elite of the Caliphate after the Abbasid Revolution and the civil wars of the 810s, both of which were won by the Khorasani troops.
While Saman Khuda might hold out, the overwhelming trend among his peers in the region would still be towards Islamization, because this was the way to get legitimacy and become part of the wider Muslim world, with all its riches and possibilities. In such an atmosphere, Zoroastrianism could only hold out in isolated, inaccessible enclaves that retained a de facto autonomy, like the mountains of Tabaristan or Sistan (or Transcaucasia, Lebanon and northern Spain, for that matter). The plains would be dominated by the Caliphate, and hence be subject to Islamization.
One way to pre-empt this would be to have the Abbasid Revolt fail quickly and the much more Arab-minded and exclusive Umayyads remain in power. The strains of holding their vast empire together were already evident before the Revolution, and it is rather likely that without the Abbasids, who used Islam as a unifying force and introduced non-Arabs to positions of power, the breakup of the Caliphate would have begun earlier. A Damascus-based Caliphate would also remain dominated by Syrian and Mesopotamian elites and be focused on the Mashriq, so less attention to the Persian lands of the East and less incentive for local rulers there to convert.