It is almost impossible for an individual to change her caste in her lifetime. It will take at least five generations to change the caste.
‘Caste becomes elevated during the fifth or the seventh generation; similarly after five or seven generations one acquires that caste of which he has followed the occupations.’
For an individual Jiva, it will take several births to change her caste. The following words of Indra to Matanga can confirm this
From the order of brute life one attains to the status of humanity. If born as human being, he is sure to take birth as a Pukkasa or a Chandala. Verily, one having taken birth in that sinful order of existence, viz., Pukkasa, one, O Matanga, has to wander in it for a very long time. Passing a period of one thousand years in that order, one attains next to the status of a Sudra. In the Sudra order, again, one has to wander for a long time. After thirty thousand years one acquire the status of a Vaisya. There, in that order, one has to pass a very long period. After a time that is sixty times longer than what has been stated as the period of Sudra existence, one becomes a person of the fighting order. In the Kshatriya order one has to pass a very long time. After a time that is measured by multiplying the period last referred to by sixty, one becomes born as a fallen Brahmana. In this order one has to wander for a long period. After a time measured by multiplying the period last named by two hundred, one becomes born in the race of such a Brahmana as lives by the profession of arms. There, in that order, one has to wander for a long period. After a time measured by multiplying the period last named by three hundred, one takes birth in the race of a Brahmana that is given to the recitation of the Gayatri and other sacred Mantras. There, in that order, one has to wander for a long period. After a time measured by multiplying the period last named by four hundred, one takes birth in the race of such a Brahmana as is conversant with the entire Vedas and the scriptures, There, in that order, one has to wander for a very long period. While wandering in that status of existence, joy and grief, desire and aversion, vanity and evil speech, seek to enter into him and make a wretch of him. If he succeeds in subjugating those foes, he then attains a high end. If, on the other hand, those enemies succeed in subjugating him, he falls down from that high status like a person falling down on the ground from the high top of a palmyra tree.
[Section 28, Anusasanika Parva, Anusasana Parva, The Mahabharata]
But there are some exceptions to the words by Indra due to some reasons. The exceptions are given in the following verse
‘Viśvāmitra, Mātaṅga, Vaśiṣṭha, Nārada and others became elevated by special penances, not by birth.’
The story of Viśvāmitra changing his Kshetriya caste to the Brahmin caste is mentioned in several scriptures. Mahabharatha contains the story around this chapter in Chaitraratha Parva of Adi Parva. Gandharva says the story to Arjuna.
The story of Mathanga rushi changing his caste from chandala to Brahmin is mentioned in Skanda Purana.
But I never came across the story of Vaśiṣṭha changing his caste during his lifetime. Which scripture mentions the story of Vaśiṣṭha changing his caste and what is the story?
