Cross-tenant mailbox migration in now in Public Preview

jkg11 You can certainly do a T2T migration using source/target identities that are cloud only. if Test-MigrationServerAvailability is confirming success, you should be good to go. That would imply to me that the ExchangeGUIDs and the x500 addresses are set accordingly (at the very least). I think the real question is, does you're organization want to support those users as Cloud-only identities or do you have a business requirement to have those identities set as authoritative on premises and sync'd to the tenant. If they need to be sync'd from on prem to the tenant, you could hard/soft match an identity on prem with the existing cloud identity, though, if the MEUs you already created aren't yet in use to access any M365 resources, you might want to just re-provision them on prem.