I've just come across this interesting piece of legal advice on whether it's possible for someone in India to change their caste. There are apparently "caste certificates", and the authorities carry out stringent checks when issuing a certificate. They don't accept self-declaration. There is nothing you can do to change your caste. Being a member of a disadvantaged caste can give you access to government aid, for instance, and for this reason lying about your caste or falsifying a caste certificate can lead to dismissal and even prosecution. But apparently in the UK we can't manage to have a single document which accurately says what sex someone is.
Can I Change My Caste?