Hello Aspirant

The registration for 12th is done in 11th standard after the admission process is completed in the college.

After 11th means you have already studied one year and you can't change it now.

However, You can do it but you need a reason to do that. Also, it is not advisable in the middle of study after completion of one class you can do it. Mostly you can do it after completion of 10th and go for CBSE board college for 12th.

I hope this will helps you.

Thanks