You can change the notice period of employees by giving a notice as per section 9A of the Industrial Disputes Act. The said notice is required to be given 21 days before effecting such change. You can also make such change by amending the certified Standing Orders, if you already have a standing orders for your company.
Under both these there involves a risk, that is if you give a notice of change as per section 9A, the employees affected by such change can object it and when an objection comes it would be regarded as an industrial dispute. When an industrial dispute is raised, the question will be on what ground it has been raised. Notice period is a subject which has no relevance in Industrial Disputes Act or any other Labour Act and what is present is that "an employer is bound to give notice to its employees when he wants to retrench his employee(s)" and there is nothing in the Act to show that an employee is bound to serve notice. In such a circumstance, the objection by the employees will sustain and employer's plea will be rejected.
Now, if the notice period is as per per a written or even an oral conditions of contract of employment, then also the issue will be decided in favour of the employees because the existing contract of employment says that the employee can leave the establishment by serving one month notice.
In another chance, if you have a certified standing orders and you wish to amend the clause of notice period, then also you need the concurrence of employees and without their consent you cannot amend it. Sometimes, the employees may agree to make an amendment with a condition that the same one month notice period would apply to those who have joined before the amendment and for every new joinee, the proposed notice of three months shall be applicable.
However, nothing above said would be applicable to employees in Managerial capacity to whom ID Act will not apply.
Regards,
Madhu.T.K