This is a shot in the dark based on limited knowledge - I'm guessing
that what happened was your old user had a different ID than the new
user, despite having the same user name. Thus, SharePoint doesn't
know what to really do with it. It sounds sort of like this KB
article:

http://support.microsoft.com/kb/896593

People with 2007 already have the migrateuser switch in stsadm, but
that might help.

Perhaps the rest of us need to learn to make an AD security group  the
owner of all things sharepoint.