so make a list of users who are still not migrated keep their samaccount (loggin name)name in text file such has
jon
Jim
sara
and run against the list of users and clear their attributes
$users=Get-content c:\users.txt
Foreach($user in $users){
get-aduser $user| set-aduser -clear msExchMailboxGuid,msexchhomeservername,legacyexchangedn,mail,mailnickname,msexchmailboxsecuritydescriptor,
msexchpoliciesincluded,msexchrecipientdisplaytype,msexchrecipienttypedetails,msexchumdtmfmap,msexchuseraccountcontrol,msexchversion
}