You should take the same steps you take regularly to ensure you don't lose
your emails, contacts, and email addresses - you do a backup.
Other than that:
"Upgrading from an earlier version of Outlook
You can install Office Outlook 2007 over any previous installation of
Outlook. As in other 2007 Office release applications, user settings stored
in the registry are migrated. If a MAPI profile already exists on a user's
computer, you typically can configure your deployment to continue to use the
profile. However, if you are upgrading from an Internet Mail Only (IMO)
installation of Outlook 2000 or earlier, you might need to re-create user
profiles."
http://technet.microsoft.com/en-us/library/cc179084.aspx#Section1
William Lefkovics
http://www.exchange2007tcr.com