Dear Vasu  you haven't mentioned the exam you are talking about in particular. But am guessing that you are talking about JEE Mains and Advanced. You are eligible for JEE Advanced only if you qualify in JEE Mains. Now because NTA conducts the examination online and all communication with you is through your registered Email and mobile no#, your contact details are connected with your admit card, score, etc. Those are the same credentials that you need to use even for JEE advanced. Its like you have a hall ticket no# from CBSE and you continue with it forever. You cannot change it.