After the June expiry, the Nifty long-term options contracts (where the expiry is greater than 3 months) will be revised from the current lot size of 75 to 50. The average closing price of Nifty (cash) will be taken to adjust the contract value. Learn more.
Alternatively, the exchange may do an M2M settlement for the 25 units being reduced from the lot size. The additional 25 units will be settled in the June month expiry in the M2M settlement normally and will not roll over to the next day.
For example, you hold 1 lot Nifty Sep long CE option at 11900 strike. Nifty closes at 12000 on the last Thursday of June. You get 25 x 100 = Rs. 2500 M2M settlement EOD. Next day your lot size becomes 50 and settlement happens normally on contract expiry.
The exchange may either do this ^ or change the strike price to price in the lot size change (in case of OTM options).
We’ll update this post once we get the confirmation from the exchange.