See: https://www.advaita-vedanta.org/archives/advaita-l/2012-July/032603.html (Hard to read. It discusses the 7 generation rule for conversion to Brahmana Varna. It also talks about court cases that have been won. So I have to study it.)
Random Reddit Threads:
https://www.reddit.com/r/IndianHistory/comments/1jpjuy0/most_likely_origin_of_caste_system_in_india/
https://www.reddit.com/r/IndianHistory/comments/1jkyczy/was_the_idea_of_caste_system_in_ancient_india/ (I’ve replied to some comments)