If you create a position with the existing lot size before the transition date, it may lead to liquidity issues or challenges in managing the effectively. This is because, once the new lot size is implemented, the odd lot will not go through if your positions do not match the updated lot size requirements. 

For example, if you have an existing position in Nifty of 1 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.