Why did the market order in the pre-open session change to a limit order?
In general, any unmatched market orders in the pre-open session (WEB) will be transferred to the regular market session as limit orders, maintaining their original time stamp. These limit orders will be executed at the equilibrium price. If the Opening price cannot be determined, the market orders will be carried over at the Closing price from the previous day. To learn more, refer to this FAQ (PDF) from NSE.