Change in notice period over email by HR

Hi All, I am working on a crontract basics in a complany. While joining in my appoinment letter the notice period is clearly mentioned as one month. Suddenly after 9 months i received an email saying "In furtherance to the Appointment Letter issued, we would like to inform you that there is an amendment with reference to clause 5.1 that your notice period has been extended for 2 months instead 1 month as per the notification received from the client." This is tolat discrimination. How come they change the notice period suddenly? As per my knowledge they cannot change the notice period of an existing employee. This goes as per the appointment letter. The below is from appointment letter "Termination by Employee, if you wish to leave the services of the company, a clear written notice of thirty (30) days has to be given to the Company/client. In case of failure to giv such written notice wiuthin the perscribed time, you are bound to make good the loss suffered by the Company and any other charges/libilities Company/Client incurs consequent to the failure to give required written notice " I work for a private IT company and i work as a contractor. i joined recently and 9 months over. Suddenyl i received an email telling my notice is extended. How come this is possible ? Please advice on the above stated