We have 35 users on Exchange 2003 on an old server. As the first step of the widely recommended double hop process to eventually migrate to MS Exchange 2013, we decided to migrate to MS Exchange 2010 first on a new server. Steps we took:
1. Updated Exchange 2003 on the old server with SP2. This server is also the AD Domain Controller.
2. Installed Windows Server 2012 in a new physical server and Hyper-V as well. Same network as old server.
3. Created Domain Controller (virtual), installed Windows Server 2012, installed AD and synched with AD in old server.
4. Created Exchange Server (virtual), installed Windows Server 2008 R2 SP1, installed Exchange Server 2010 SP2.
5. Migrated mailboxes from Exchange 2003 to Exchange 2010 using steps as per http://gallery.technet.microsoft.com/exchange/Rapid-Migration-Guide-from-7ade7012
6. Transferred Global Catalog Server role from old Domain Controller (Windows 2003 Server) to new virtual Domain Controller (Windows 2012 Server).
7. Shut down old Windows 20003 Server/Domain Controller/Exchange 2003 Server.
8. Assigned IP of old Windows 20003 Server/Domain Controller/Exchange 2003 Server to new Exchange 2010 Server.
9. Tested: mail flow send/receive internally in the network is successful.
10. Tested: mail flow send/receive external to the network (through internet) is not working.