You can look at Exchange 2003 accounts via the 2010 Management console but is modifying supported?
No warnings that it is not, and all is held in Active Directory.
Adding an additional email address works...
But results in Error 4, Keywords "classic"
Task Get-MailboxStatistics writing error when processing record of index 0. Error: Microsoft.Exchange.Management.Tasks.MdbAdminTaskException: Mailbox 'domain/OU/account name' doesn't exist in an Exchange 2007 or later mailbox database.
Management Console has the updated change, as does ADUC in 2003.