Do you plan your M365 migration by mailbox count, and treat identity as a pre-requisite step?

Data volume was the biggest challenge three years ago. Mailbox size, SharePoint Content, OneDrive Storage. These are all manageable issues with the right tools and a pre-staged pass structure that moves data in bulk before the cutover window is opened.

Identity is a separate category of problem. It’s not a problem of volume. It is an architectural problem.

Entra ID is now the most complex, and therefore highest-risk, component of any tenant to tenant migration. The precision needed at the identity layer differs from the parallel data migration work. While simultaneously running server and file server migrations using a purpose-built tool for identity migration, it is necessary to create a sequencing model which accounts for the downstream authentication dependencies at each step.

SSO dependencies are broken when the source application registration has been deactivated prior to the destination registration being validated. Hybrid identity configurations fail when the timing of Active Directory synchronization on premises is not coordinated with Entra ID’s cutover. Domain rehoming breaks authentication chains in any application that hardcodes the old UPN domain into its trust configuration. Mismatches in OAuth and SAML protocols between source and destination tenant settings can cause failures that appear to be tool errors, but are actually identity architecture failures.

The job market has responded to this. As of now, M365 Migration specialist roles require Entra ID restructuring and identity consolidation as well as hybrid identity configuration. Not preferred. Required. This shift in language reflects the fact that many migration programs have struggled or failed at the identity layer, after having succeeded at the data layer.

It is becoming increasingly difficult to locate migration engineers who have successfully run Entra ID object-migrations at scale along with data migration workstreams. This is where the premium for specialization in this market is concentrated.

What is the order of your identity migration sequence in relation to your mailbox migration timeline?

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