Office 365 Migration - Azure AD before Mailbox migrations?

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Hi guys,


A couple of quick questions regarding a cut-over migration to Office 365.


A small number of staff here in the office (total 7), have been using Office 365 for the past year. Mostly for Sharepoint & SFB, and with no on-premise AD Sync. We're purchasing new licenses, and we now what to move all users (approx 15 in total) from our on-premise 2013 to Exchange Online, and merge/sync our on-premise AD user accounts with Azure AD using Azure AD Connect. We've been doing some research, and it seems like a cut-over migration is the simplest way to go for our environment. My two questions are:

1. For a cut-over migration, is it better to migrate the mailboxes to Exchange online first and then configure/sync Azure AD Connect.

2. What should we do with the current 365 accounts? We would like to be able to merge these current 365 accounts with the corresponding on-premise identities that are to be migrated/synced (e.g. We want to merge the 365 account  with on-premise AD account). Is this possible with a cutover migration? Or do we need to delete the current 365 accounts, to be able to migrate the on-premise mailboxes? 

Any info or insights would be greatly appreciated.




2 Replies

Hi Brendan,


1. if you are decide to use a cutover migration, there will be a kind of one-time sync for your on-premises identities to Azure AD. You have to disable directory synchronization in your tenant prior you can start migrate your mailboxes. Important hint: if you need directory synchronization after migration, you cannot decommission your Exchange on-premises server. Some attributes are set to read-only in Exchange Online if directory synchronization is enabled. And the current supported attribute change, i.e. for proxyaddresses, is to use an on-premises Exchange server. This means you cannot get rid of your on-premises infrastructure.


2. AFAIK you cannot migrate to existing users, it will throw an error during the first synchronization. You can merge it with Azure AD Connect but then you cannot use a cutover migration.




Thanks very much for the response Dominik. It's exactly what I was looking for.