Hi,
Management has every right to amend the policies at any point in time, including the notice period, but the same should be circulated to all employees. When it comes to the notice period, yes, they can change the period and terms, but the same should be intimated to all employees. A letter should be drafted, stating that the notice period is changed from one month to two months, and HR should obtain the signature from the employee. Only then does the two-month notice period become valid and come into effect.
Talk to your management and get it settled.