Policy Communication and Employee Agreement
Any new policies introduced by the management must be brought to the notice of the employees through official circulars or memos, etc. However, when employees sign the employment contract, there is a clause that mentions that the employee abides by and agrees to all the policies regarding his/her employment with the company.
Any policy introduced by the management has to conform to local labor laws. As members of the firm, every employee is bound to follow the policies set or introduced by the management within the purview of the law.
So in your case, if your management has changed the notice period policy from 1 month to 3 months and this change is communicated to all employees through official channels, then it becomes valid and binding on employees to follow this policy.
However, if an employee has already resigned, their resignation has been accepted, and the notice period is in progress, and then the management amends the notice period policy and imposes an extension, then it is not valid and binding on the resigned employee.
These are my opinions. However, let's hear from readers and seniors in this context.