Alright, a bit weird, but hear me out:
The reason the people of Northeast India are Christians is because they were cut off from the cultural influences from the rest of India. As such they developed their own pagan religions but they were eager to accept Christianity.
Let's say some Parsis in India are a bit sad how far their religion has fallen and decide to make a missionary movement (similar to Arya Samaj by Hindus or Tableeghi Jamaat by Muslims) to convert people to Zoroastrianism. Some missionaries go to the northeast and achieve successes. They report back to the rest of the Zoroastrian missionary movement leading to more missionaries to go there and soon the northeast gradually becomes Zoroastrian.
How does this impact India, in the long run? How is Zoroastrianism affected?