Not only do you want to reverse the output pwm values, you will also want to swap the left and right side
At the "swamp thing" scrimmage a few weeks ago, we decided to swap front and back in between rounds. After simply reversing the output pwms (255-output pwm), we found out much to our driver's amusement that not only were front and back reversed, but so were left and right
....it is also a good idea to "min/max" the outputs to stay between 1 and 254
eric