One cannot change ones caste. For all practical purposes, legally, caste of a person makes no difference unless you want to take the benefit of reservations. Only for SCs and STs (Scheduled Castes and Scheduled Tribes), castes matter, legally.
All others are open category.
Secondly, you cannot be an Atheist legally. And let me tell you why. It is not because India doesn't recognize Atheism. India does. Freedom of religion contains freedom to not follow any.
The reason is simple -
Because India still has different personal laws for different religious people. Hindus, Muslims, Parsis and Christians all have different laws. And Atheists don't have a secular law to govern them.
While we do have Special marriage Act for marriages among atheists or people belonging to different religions, we do not have other provisions relating to succession for Atheists.
So, even if you are an Atheist, you will have to politically identify yourself as a person belonging to your birth or converted religion so that the state can apply the respective law to you.