In hue and cry company notice period policy changed and enforced w.e.f from the day mail came from GM's desk. At the time of joining, notice period was 1 month and now one fine morning you reach office and find that it has been changed to 3 months effective from same day.

The offer letter doesn't mention anything about Notice period being subjected to change but the management claims that it is totally legal. Very sure that they are bluffing.

2 questions:

1. Does the company has jurisdication to change the Notice Period even it is not mentioned and without asking employees to sign the change.
2. Most importantly, how legal is it to enforce it on the very same moment the mail was sent from GM desk?