Hi,
You probably should read the system documentation for your release, specifically the sections on CIFS administration.
Basically, you just need to re-run 'cifs setup' to change your domains. You need to be the "root" user and have an AD user that has enough permissions to add machines to the domain and some information on what container they will go into, etc.
The real problem starts if you haven't migrated your users as-is (no SID changes) to the new domain. You then have to go through and change the rights and ownership on all of your files to reflect the changes in user and groups.
Good luck.
