Have you tried running the old STSADM way of migrating users - like the following script.
STSADM –o migrateuser –oldlogin MYDOMAIN\old –newlogin MYDOMAIN\new –ignoresidhistory
There might be orphaned users which doesn't get handled correct. You can clean up these orphans using the following script:
# Clean up accounts that is not imported correctly
# Ref: http://iedaddy.com/2012/02/sharepoint-2010user-information-lists-and-user-profile-cleanup/
$upsa = Get-SPServiceApplication | Where-Object {$_.TypeName -like "User Profile Service Application"}
# List all user accounts that is not imported correctly
Set-SPProfileServiceApplication $upsa -GetNonImportedObjects $true
# Remove user accounts not imported correctly
# Uncomment line below to run
Set-SPProfileServiceApplication $upsa -PurgeNonImportedObjects $true
# Run a full User Profile Service Syncronisation, and make sure users end up in Profile database.
# If not, users who log in will create new NonImportedObject accounts
When this is done you need to run a full syncronization of you User Profile Service Application. These steps worked for me when the Active Directory changed and we needed to map users and groups of the new AD with the old AD.