Either use Quest Migration Manager for AD: http://www.quest.com/migration-manager-for-active-directory/
Or a mixture of STSADM and Powershell.
# extract all users into an xml file
[xml] $users = c:"program filescommon filesmicrosoft sharedweb server extensions12bin"stsadm.exe -o enumusers -url http://wsstestsite
#iterate through each user (note the dot notation for accessing nodes)
foreach($login in $users.Users.User)
{
# determine old and new login (using regular .Net string func)
$oldlogin = $login.Login
$newlogin = $login.Login.Replace("LocalMachine", "Domain")
# migrate user
c:"program filescommon filesmicrosoft sharedweb server extensions12bin"stsadm.exe -o migrateuser -oldlogin $oldlogin -newlogin $newlogin
}