How do you migrate SharePoint Portal Server user accounts after a domain change? First of all, make sure that either the post-SP1 hot fixes mentioned in the KB articles below or even better, Service Pack 2 for Windows SharePoint Services and SharePoint Portal Server 2003 have been installed. And then....

1) Run stsadm -o migrateuser -oldlogin DOMAIN\user -newlogin DOMAIN\user [-ignoresidhistory] for a user as described in http://support.microsoft.com/kb/896593/

2) Create an application that calls Microsoft.SharePoint.Portal.PortalAccountMigrationManager.MigrateAccount() as described in http://support.microsoft.com/kb/896161/

3) Repeat steps 1-2 for each user you wish to migrate

You could certainly take advantage of SPUserUtil, have a look at http://blogs.msdn.com/krichie/archive/2006/02/18/534752.aspx