Which 8 speed do you have? If memory serves (and I'm probably remembering incorrectly) 8 speed Dura Ace rear derailleurs had the same pull ratio as 9 speed rear derailleurs (which would mean you wouldn't need to buy a new RD, and you don't need a new FD unless you're going from 2x to 3x or standard to compact cranksets) but other 8 speed did not.