it can be rectified. my friend filled the wrong registration number and it was rectified. so here is what you should be doing.
1. write a letter to icai about the mistake you have made.
2. hopefully you have a photocopy of the form you send to the insti.
3. highlight the field where you have made the mistake in the form and attach it with the handwritten letter.
4. in the letter state the form bar code no, your registration number and clearly state the mistakes and what should have been the correct thing.
i think your CA had to sign that form... ? dint he read it before signing ?
and yeah do call the institute helpline about the mistake before doing what i have said.
what i have told is something that my friend did when he made a mistake in the OMR sheet.