Hello student sorry to inform you but no this is not possible to change board in class 12 as the registration for class 12 already submitted to cbse in class 11. If you want to change your board then you need to take admission again in class 11 In cbse school which is a huge loss of 1 year so I won't suggest you that. Please complete your class 12 education from the same board and then further take admission according to you choice.

Feel free to comment if you have any doubt

Good luck