Changing service accounts in the SharePoint 2003 used to be a nightmare, as it was all done manually, and you had to find every instance through the interface and change them all at the same time. It seems Microsoft have thought about this more in the new version and have included it as part of the command-line processes. Then they have documented the process as a KB article
here . This is useful if any of the service accounts have expired or the SQL account is no longer valid.