As per section 9A of ID Act, a proior notice of 21 days should be given to the worker/employees for the effect of any change in the service conditions in respect of any matters prescribed in Schedule IV appended to the Act.
There are 11 ground which has been mentioned in the IV schedule. As per me yours act of increasing notice period form 15 days to 2 months falls with the clause 8 withdrawal of any customary concession or privilage or change in usage. or clause 9 introduction of new rules of discipline or alteration of existing rules, except in so far as they are provided in standing order.
so in view of this a notice of 21 days is mandatory to change the resignation/termination notice period from the day of issuance of notice before effecting change.