No, modifications to existing will not be allowed once the updated lot size takes effect. But you will be able to add the new lot size. 

Please ensure that you take appropriate action before the specified date. For example, if you have an existing position in Nifty of one lot i.e., 25 quantities, and the new Nifty lot size is 75 quantities. Post the transition date you will not be able to  modify or add odd lot quantities in denominations of 25 or 50 but you can always buy a new lot of nifty where 1 lot is equal to 75 quantities