As per Section 9A of the ID Act, a prior notice of 21 days should be given to the workers/employees for any changes in the service conditions regarding matters specified in Schedule IV attached to the Act.
There are 11 grounds mentioned in Schedule IV. In my opinion, your action of increasing the notice period from 15 days to 2 months falls under either clause 8, which pertains to the withdrawal of any customary concession or privilege, or change in usage, or clause 9, which relates to the introduction of new rules of discipline or alteration of existing rules, unless provided for in the standing orders.
Therefore, considering the above, a notice period of 21 days is mandatory to modify the resignation/termination notice period from the date of issuing the notice before implementing any changes.