Employer is forcing to serve 90 days notice period which was 30 days when joined the company

Hello sir, I have recently resigned from US mnc and have requested HR and director to relieve me in 30 days notice period which was mentioned during my confirmation of employment. I have been working in the company more than a Decade and even have asked HR that I will arrange ready person who can take my responsibility for job within 15 days then also stated on mail to deduct my salary for not serving notice period- 90 days which was amended during my long spam of career with this company. After 5-6 years, new document which was amended to 90 days and was mailed to all employees who were in this old terms 30 days notice period. Everyone has accepted to since it was company policy as HR mentioned and all have to follow but was without any verbal communication or discussions. Now company is asking me to serve 90 days notice period or they will deduct my salary and will.adjust that in full and final settlement if I don't serve. Request to please suggest . Thanks and regards Mahavir solanki