No if you have passed the examination and if you are awarded the certificate based on your registration and preference for 10+2 , you can't change the subject type based on the same result. Also having additional subject and main subject doesn't matter as for most higher education courses best of 5 is considered. Also if you are going for higher education you have no need to change your 10+2 subjects. See if you need Mathematics as a subject of eligibility then be it Additional or compulsory doesn't matter, you are eligible for the purpose.

I hope this answer helps. All the very best for your future endeavors!