From: Reetu on
Hi All,

We have a parent domain called 'A' and have two child domains called 'B' and
'C' in the same forest. We are planning to rename the parent domain as "root"
and move all the users and groups from domain B and C to "root" domain A. We
will be using ADMT to move the users and the SID history will be retained.

Since SID history will be the same, user names will be same and the move is
in the same forest I am assuming there should not be be any issues for the
users to access SharePoint site collection, sites, lists etc. The users are
added as B\user1, C\user2 etc. Are there any missing steps that I need to
take care of so that the users can access the site like before.

I appreciate your help.

Regards,
-Reetu
From: "Stephen Woodard[MS]" on
SharePoint maintains its own list of user accounts including the domain
they came from. For a smooth transition between domains you will want to
run the migrateuser command after you move the accounts.

http://technet.microsoft.com/en-us/library/cc262141.aspx

-Stephen