Most of the newer Shimano stuff is 9- and 10-speed compatible. You'll have a part number on the back of the RD - check the shimano website for specs. I know because I'm making this upgrade as we speak - I had some unfortunate circumstances which rendered my 9 speed shifters unuseable so I seized the opportunity to upgrade.

Best of luck.

Why might you have to change the chainrings? Different width chain? One of the things I've noticed since I made the upgrade is the chain is somewhat noisy on the big ring. Is this symptomatic of needing new rings?