Change user account in site collection

We have added a second domain with it's own set of user into our SharePoint 2007 environment.  What I would like to do is replace anywhere a certain user has been given permission in a site collection with a different user (its the same person but they have two user account).  I have tried the migrateuser stsadm command, but this just appears to replace one account with another in the user import (this doesn't help as both are in there anyway!).

Is there a way that I can achieve this?

thanks