Reversing PWM output
Could this be established by simply writing a value of (255-Value)-for 8bit PWM?


(255 - value) is the best option as it involves no more hardware.



Figure 1. PWM signal transitioning from high pulse width (75%) to low (25%) and back again. Note amplitude remains constant.

PWM switches between V+ and GND. If you invert the logic it will still switches between those two voltages.


  ... or should a NAND chip or other method be used?


If the problem had to be solved without access to the firmware then the NAND gate (wired as an inverter) would be a good solution.