MS Outlook Contact detail updates

Running Exchange Server 2000. (Upgrading soon). I have changed some users “Display name” in AD Users and Computers. So instead of just Martin Hahmann in the display name, it is now “Martin Hahmann (IT Manager)” as an example. How can this be updated to all users, using Outlook 2003, when they send a new email, as now the old name is still being used.