The Adapter Migration Toolkit allows you to reuse your Microsoft BizTalk Server 2002 custom components (AICs, preprocessors) in a BizTalk Server 2004 environment. See here for details:

http://www.microsoft.com/downloads/details.aspx?FamilyId=3383F89E-8223-4DB5-947A-1873C4C555BB&displaylang=en