0365 Migration

Hi, I have inherited a bit of a mess and want to walk through some things about an 0365 Migration.

First, the client has an on-Prem Exchange server that they use for one of their primary domains.  They also have an 0365 tenant that some of their users in other locations use.  (These are different domains than what is on the OnPremise Exchange server.)

As part of the O365 tenant, they have an instance of Azure AD.  This is not synced to the onPremise AD.  However, some of the users are in both, Azure and onPrem AD.

What I am struggling with now is the best approach to get them fully migrated from this scenario to O365.  I figured I need to do the following:

* Get Azure and onPrem AD Syncing
* Ensure all the PreReqs for Exchange are done
* Setup Certificates
* Setup a Hybrid between the onPrem Exchange and 0365.

My question is:

* What is the best way to get AD Synced.  Obviously, I will use the Azure AD Sync tool but, will that overwrite data that is currently in Azure?  Or will it simply create another account for the user and just treat it as a Local AD Account and not an Azure AD Account?
* Could in also impact the accounts in the onPrem AD?  Or will it only sync the accounts in the OnPrem up the Azure AD?

* Can I make a hybrid of a domain that is not Primary in O365?  The domain of the OnPremExchange is not the primary domain in Azure AD.  Will that be ok?
* Will this have any impact on the mailboxes or domains that are different but are already in O365?

I am just a little confused on the best way to approach the migration given they for the most part already have an Org in O365 and Azure AD that is not aware of the onPrem AD and Exchange.

Once I get it migrated, the plan is to make the primary domain all the same for each mailbox.

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Vasil Michev (MVP)Commented:
For objects that are already existing in O365, you can use the so-called "soft-matching" mechanism as described here: http://support.microsoft.com/kb/2641663

No impact on on-premises accounts.

Yes, you can create Hybrid with any custom domain.

The impact will depend on what exactly you configure. Hybrid involves connectors and changes to the mail flow in general, but as long as it's properly configured it's transparent to the end users.

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