It is not clear that confusion is about the medium of writing examination or changing the optional subject. After spending thousands of Rupees in coaching all you gained is that you realized that you are more comfortable with Hindi medium.
We suggest that instead of picking a new subject, you should build up the knowledge you have gained with the optional subject Public Administration.
Whatever you have read would be handy even if you opt for Hindi medium and as such you can take help from Abhay Kumar who teaches Public Administration in Hindi medium at Synergy.
Still, if you wish to take a new optional, History would be a better bet in Hindi medium as a lot of books, study-material and even quality guidance is available in Hindi medium.