I'm not a MAC user myself so I wouldn't risk offering specific advice.

CNET has a number of MAC forums and the contributors there would be much better placed to assist you;
http://forums.cnet.com/mac-forums/?tag=f06-fd;StickyWin_1264072583290

However, at the risk of making a fool of myself I would say this. We only lose what we don't backup, and in particular, at least on Windows' systems, upgrading from one OS to another is fraught with difficulties. So I would suggest that you backup what you can before the upgrade.

I have found a couple of Microsoft web site pages about exporting and importing Entourage information, and perhaps they will be helpful to you;

Export or back up information

Import information into Entourage.

I hope that is useful, but if not, please do visit CNET's Mac forums. I hear they don't eat too many people; Devil

Mark