Thanks, life is never as simple as you'd hope. They are a CC4 user and want to migrate to a vanilla MS environment too.
Based on what you say does it make sense to move from AD sync to Dirsync initially to remove Unify from the equation and then look at the domain migration?
Migrating between domains with dirsync seems reasonably straightforward by deactivating dirsync within Office365 effectively isolating those user accounts from the local domain. Then populate the new domain with the users and attributes (userPrincipalName, proxyAddress, LegacyExchangeDN). As the user GUID will change then the immutableID will need clearing out of Azure - after the new Dirsync is setup, O365 will then SMTP match users based on primary email address and create a new ID in Azure AD.
Does that make sense to you?
Thanks