Upgrading to new hydraulic brakes - should I put same sized rotors on front and rear wheel?
This is a common set up, as the majority of your stopping power is assumed to come from the front wheel, and many frames, especially older frames, had less clearance on the rear triangle of the frame, so often the 180mm  rotor wouldn't fit. 

You can change it if you like, but there is little necessity to do so. 

If you choose to change it, you will only need a rotor, and the appropriate adaptor for you brake caliper type.