I need to find Vin max(Common Mode) and min(common mode) for given circuit.
Yes you can change it to a current source but be prepared to consider what happens when \$V_{DS}\$ gets too low as this will affect that current: -



In the picture above the current remains fairly constant above a \$V_{DS}\$ of about 2 volts. Note also that there is a slight slope to the constant current (saturation) area and this may be taken into account in your analysis by using a high value resistor in parallel with the current source.