As you should well know Exchange 2003 will end support on april 2014.
If you want to be updated al Microsoft Lifecyle produt you can check this official Microsoft link:
http://support.microsoft.com/lifecycle/default.aspx?LN=en-us&c2=730&x=15&y=9
The Exchange 2003 Migration Tool Kit
Exchange Server Deployment Assistant:http://technet.microsoft.com/en-us/library/ee681665(v=exchg.141).aspx
Exchange 2003 - Planning Roadmap for Upgrade and Coexistence
http://technet.microsoft.com/en-us/library/aa998186(v=exchg.141).aspx
here are some references:
References
Understanding Upgrade to Exchange 2010http://technet.microsoft.com/en-us/library/aa998604(v=exchg.141).aspx
Understanding Upgrade from Exchange 2003 to Exchange 2010
http://technet.microsoft.com/en-us/library/ff805040(v=exchg.141).aspx
Understanding Upgrade from Exchange 2003 and Exchange 2007 to Exchange 2010
http://technet.microsoft.com/en-us/library/ff805036(v=exchg.141).aspx
Checklist: Upgrading from Exchange 2003 and Exchange 2007
http://technet.microsoft.com/en-us/library/ff805038(v=exchg.141).aspx
Upgrading Exchange Server 2003 to Exchange Server 2007
http://technet.microsoft.com/en-us/library/hh757966.aspx
Common Mistakes When Upgrading Exchange 2000/2003 To a Exchange 2007
http://support.microsoft.com/kb/555854
Upgrading Exchange ActiveSync to Exchange 2010
http://blogs.technet.com/b/exchange/archive/2009/12/08/upgrading-exchange-activesync-to-exchange-2010.aspx
MCS team blog: Prepare client side environment to Upgrade from Exchange 2003/2007 to Exchange 2010
http://blogs.technet.com/b/meamcs/archive/2010/12/19/prepare-client-side-environment-to-upgrade-from-exchange-2003-to-exchange-2010.aspx
Upgrade from Exchange 2003 Mailbox
http://technet.microsoft.com/en-us/library/dd638113(v=exchg.141).aspx
How to Remove the Last Legacy Exchange Server from an Organization
http://technet.microsoft.com/en-us/library/bb288905.aspx
How to remove the first Exchange Server 2003 computer from the administrative group
http://support.microsoft.com/kb/822931
For SBS customers: Guided Walk-Through of How to Remove the Last Legacy Exchange Server from an Organization in a SBS 2003 to SBS 2011 Migration
http://social.technet.microsoft.com/wiki/contents/articles/2263.remove-the-last-legacy-exchange-server.aspx
How to Uninstall Exchange Server 2003
http://technet.microsoft.com/en-us/library/bb125110(v=EXCHG.65).aspx
How to migrate mailbox data by using the Exchange Admin Center in Office 365
http://support.microsoft.com/kb/2798131/en-gb
Outlook.com Help | How to perform an Exchange 2003 cutover migration to the Office 365 Cloud
http://help.outlook.com/en-us/140/ms.exch.ecp.emailmigrationwizardexchangelearnmore.aspx
Outlook.com Help | How to perform an Exchange 2003 staged migration to the Office 365 Cloud
http://help.outlook.com/en-us/140/ff959224.aspx
Microsoft Product support lifecycle information by product family
http://support.microsoft.com/lifecycle/default.aspx?LN=en-us&c2=730&x=15&y=9
