If you go online with the PLC you should be able to Upload it's program to your computer. Remember, the PLC exists in a level "lower" than your computer - so you Download to send programs "down" to the PLC, and upload to move programs "up" to your computer.
As far as how to do a comparison - I too believe there is a way to do this, but I'm not sure how as I'm still learning myself!
Good luck.