I ve found this guide on move: Upgrade all Exchange Servers to Exchange Server 2007 Service Pack 2. Bring the AD forest and domains to Windows Server 2003 Functional (or higher) levels. Upgrade at least one Global Catalog domain controller in each AD Site that will house Exchange Server to Windows Server 2003 SP2 or greater. Prepare a Windows Server 2008 (RTM or R2) x64 edition server for the first Exchange 2010 server. Install the AD LDIFDE tools on the new Exchange 2010 server (to upgrade the schema). Install any necessary prerequisites (WWW for CAS server role). Run setup on the Exchange 2010 server, upgrade the schema, and prepare the forest and domains. (Setup runs all in one step or separate at the command line.) Install CAS server role servers and configure per 2010 design. Validate function-ality. Transfer OWA, ActiveSync, and Outlook Anywhere traffic to new CAS servers. Install Hub Transport role and configure per 2010 design. Transfer inbound and outbound mail traffic to the 2010 HT servers. Can I get to point 10 without exchange 2010 affecting 2007 server?

So you would like to make sure the Exchange 2007 server works all the time before step 10? Basically it will be. However, I would suggest you schedule a downtime in step 9 you transfer OWA,Outlook anywhere and ActiveSync to the new 2010 CAS server. Refer to: Transitioning Client Access to Exchange Server 2010 http://blogs.technet.com/b/exchange/archive/2009/11/20/3408856.aspxFiona