Distributing CPUU on non-ad machines


Im working on a case on which I am proposing Ondemand Migration for Email. I have some questions regarding the product and I was hoping someone could help me. I also have an assumptions and I was hoping to clarify them here.

1. What would be the best approach to distribute CPUU on non-domain joined machines? Some mergers are only between 2 Office 365 tenants where there is no Active Directory, only an Azure Active Directory. Ive used CPUU before in AD environments which was easy to to, but now I dont have the ability to use a start up script. Maybe someone can share their experiences on how this was achieved? 

2. This is an assumption, I was hoping someone could confirm this.. Im guessing the switch message is send when the mailbox is migrated? Or is the switch message send when you enable the CPUU setting to update? What my idea is, is to first run the migration without the CPUU setting enabled, then re-run the migration with the CPUU setting enabled. This way I can migrate most of the mailbox content without switching the mailbox. When ready to switch run the delta sync and switch the mailbox. Is this a scenario that is supported?