You can put an 8, 9 or 10-speed cassette on the same hub (wheel). They're the same width. But to fit more sprockets on the same width, each sprocket gets narrower and the distance between them gets less as you go from 8 to 9 to 10. Therefore you will need a specific 8, 9 or 10 speed chain to fit the sprockets.
The rear derailleur doesn't care how many sprockets you have, it just moves as much as the shifter on the bars tells it to. The rear shifter, on your bars, does care. It's indexed to 8, 9 or 10, and if you want to swap your cassette to one with more sprockets you also have to swap your shifter.
With my limited experience of road bikes, and if I have understood Sheldon Brown and my LBS correctly.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett