The NOW Trading Terminal has an inbuilt option to convert You Open Positions .
You can manually do this, if you wish to convert from MIS to NRML / CNC or vice-versa.
The option to convert positions are available in your TradeBook Menu.
To open the TradeBook Window, you can Press the Shortcut key F8 or using the Orders and Trades Menu on the toolbar.
On the Trade Book Window. Select the order you want to convert and click the Position Conversion button at the bottom.
Click OK to confirm. Simple as that.