Bryan Krieger has put together a very thorough document on upgrading to the RC: http://blogs.msdn.com/bkrieger/archive/2010/02/03/tfs-2010-beta2-to-rc-upgrade-guide.aspx

It's kind of long because it covers almost every form of upgrade you might consider.  For most people, upgrading will be uninstall Beta 2, install RC and run the "Upgrade from Previous Version" wizard.  If you have build controllers/agents, you'll need to uninstall and reinstall them as well.  There's tons of good detail in the doc that should answer just about every question you might have.

It also covers bugs that we know about in the RC.  The most painful of which is probably the fact that the RC Proxy doesn't work.  There was a late change to some setup logic that caused the proxy to break and by the time we discovered it, it was too late to fix it for the RC.  You can continue to use the Beta 2 proxy if you need one.  Of course, the proxy will be fixed again for RTM.  There are a few other bugs documented in Bryan's document.

Let me or Bryan know if you have any questions.

Brian