Manish, No. Your Maths subject will be valid and so will be the marks you scored. That will always be honoured even in the marks list. That will not be taken away from you or your marks list by any chance. Now you are studying Biology as an additional subject to qualify for NEET. That doesn't disqualify on null the Mathematics you studied in your 12th or the marks you scored. You can still use them for admissions where Mathematics is necessary.