how to change Administrator mailbox Exchange 2003

We have migrated from an NT 4 domain to a new Windows 2003 Active
Directory domain. We have an Exchange 5.5 and Exchange 2003 site with
ADC running between the mail servers. ADC created an account for the
old domain Administrator (which has a mailbox on the 5.5 server) in the
AD Recepients container (and disabled it) and associated the Exchange
5.5 mailbox with it. The new domain already has an Administrator
account (built-in account) in the User container.

How can I associate the mailbox with the enabled Administrator account
in the Users container, and get rid of the disabled Administrator
account in the Recepients container?