hi Bro , it is not at all a problem since ypu have to place a seperate application for the again april JEE exam no you can definetly change your caste and your details and apply for it , but you should definetly have a copy of those certificates like OBC which was given by the state government  and it should be valid too since you have to scan yhe copy and submit it to fill the procedure of JEE mains , mothers occupation is not at all a problem but if your mother is working in a defence and armed forces then it has a good value in getting a good seat and a good college so dont worry about it bro , you should study well and smart but not hard and success will be definetly yours so all the best for the comming exams Bro