Hello Aspirant.

Yes you can change your subject from PCMB to PCB in your class 12th as the final registration for the board exam is done in Class 12th.

For this you have to consult with your college management to make changes in your class 11th registration form but once the registration for class 12th is done there is no chance of making changes so consult with the college management as soon as possible.

All the best.