Hi Aspirant

In most of the CBSE/Other equivalent boards school/colleges usually registration for class 12 is done when you are in class 11.So if you have already filled up the registration form you might not be able to change your subject.If you haven't filled the form yet you can ask your teachers about it.

When you are in 12 ,you only need to fill form to appear in final exam not the registration form.So you cannot change subjects because you have already registerd the subjects of your choice.

All the best.