Migrate from Exchange 2007 to Exchange 2010

We currently have Exchange 2007 SP2 running on Windows Server 2003 x64. We are planning on upgrading to Exchange 2010 running on a Windows Server 2008 R2. I have verified that our new server meets all Exchange 2010 requirements and our domain function level is Windows Server 2008.
First I am going to install the client access role on the new server. We do have users that connect via Outlook Web Access so I need to configure the internet facing domain name. My question is do I put in our current internet facing domain name in here or do I create a new one?