You do not need a new RD. You need new shifters, cassette and chain for going to 9 speed. I went from my old downtube 7 spd shifters to 9 speed with just a new cassette and chain. The friction mode on the downtube shifters worked just fine with 9 spd. Of course, later on I swapped them for STI, but that is a different story. In fact, even an older mtn bike rear derailleur will work fine.