Threatened reversal of approved designation after resignation, what are my legal options?

I work for a private company in Tamil Nadu. I originally worked in electrical design for a year after joining, but since 2022 have worked full-time on software development and AI/ML projects, without managerial authority. My designation was changed to “Data Analytics Engineer” following my supervisor’s request and GM’s email approval. It appears in my bonafide letter, SAP and attendance records. After I resigned, the GM threatened to damage my career and give adverse feedback. My supervisor subsequently informed me that she intends to revert my designation to “Design Engineer” and use that title in my experience letter. I have already declared my approved designation to my prospective employer and am concerned about background verification. Please advise: Can this approved designation be reversed during notice, and how can I challenge inaccurate exit documents or verification responses? Which grievance/labour authority or legal remedy may apply? What should my written objection contain, and when should I engage an employment lawyer? I have the approval emails and supporting employment records.