Hello! In the case of CBSE, students who appear for improvement exams usually do not get a fresh migration certificate again. The migration certificate is normally issued only once, in the year when you first passed class 12. For improvement candidates, CBSE only provides the updated marksheet but not a new migration, even if the roll number is different. So, you can use the migration certificate that was issued to you in your original passing year along with your improvement marksheet for admission. If the college insists on a new migration, you may apply to CBSE regional office with a request, but in most cases the old migration is valid.