MSExchangeTeam blogged about OWA transition from previous version to Exchange Server 2010. Good stuff!
Many of you have been asking how you can transition your existing Exchange environment to Exchange 2010 from a client access perspective. For most of you, this will also mean coexisting with legacy Exchange and Exchange 2010 for a period of time. My previous blog articlediscussed the overall steps in how to upgrade your environment from a client access perspective. This article will discuss how Outlook Web App will function in an Exchange 2003 or 2007 environment that has Exchange 2010 deployed.
Read more at source: http://msexchangeteam.com/archive/2009/12/02/453367.aspx