If your company is making any changes in terms mentioned in your appointment letter, company shall have to ask you to give your acceptance.
If not acceptable, you can decline. The changes shall not be applicable if not accepted by you.
If you have any queries you can write to your appointing authority, MD, or the employee who has sent you the
Usually companies resort to change in notice period when they need to check attrition or deter the employees to leave, or they wish to retain employees for longer period so that there work is not affected and they have more time to find replacement, and they want to increse the amount of recoveries,and when they have tested most of the employees and have decided to replace.