DA 9 speed is SIS compatible, so any SIS components like your current RD will work fine with a DA shifer, or DA 9 speed cassette. DA 8 Speed and below is not SIS compatible, so if you use an 8 Speed DA cassette, RD, or shifter, you'll have to match all the other parts with pre 9 speed DA parts. In other words you are better off going with 9 speed. I wouldn't upgrade the RD unless you really want to. I have a bike that came with 6 speed DA. When the freehub body went bad I took it as an opportunity to upgrade to 9 speed. I used an 8speed DA freehub body which fit on my older hub, and an Ultegra 9-speed cassette which fits on the 8-speed freehub, 9-speed DA downtube shifters and a 600 tricolor RD which was originally used with an 8-speed SIS group. As long as everything is SIS, everything should work great together.