Install ADC to synchronize the Exchange Server 5.5 directory with AD.
Coexistence allows Exchange Server 5.5 and Exchange Server 2003 to be part of the same organization. Coexistence allows you to move users from Exchange 5.5 to Exchange Server 2003 with the Move Mailbox tool.
To migrate from an old Exchange Server 5.5 organization to a new Exchange Server 2003 organization, use the Exchange Server Migration Wizard.