Noc required from ca, but he refuses to

Dear Members

I am looking for a answer to very specific question.

Client: ABC - Sole proprietor

Legal Advisor - XYZ

Auditor - PQR

Now the situation is that ABC approached XYZ for Tax Audit purpose, so he took up the assignment and got the ABC's Book of account audited by PQR. ABC Gets report through XYZ and paid fees to him. As the business of ABC has increased significantly, he is not satisfied with his auditor and want to appoint M/S RKS. RKS when approached told ABC to get No Objection Certificate (NOC) from Previous Auditor. XYZ refused to give NOC and threated client that he will sue and make sure his business premises gets raided by Income tax department.

What are the appropriate action that can be taken by ABC to change is auditor?

Is there any procedure by which RKS can be appointed as Tax Auditor