Hybrid deployment : Exchange 2010 to Office 365

Which type of migration are you aiming for? Guessing Staged or Hybrid? Both allow you to move the mailboxes in batches, so you can easily do a test run with a user or two.

The licensing part is a bit unclear however, do you mean that you have already applied Exchange license to the cloud user, thus provisioning a mailbox for them?

It is an Hybrid migration with the Exchange certificates that I already imported (sorry for not being clear with the "licences" term). As I stated previously, I wanted to migrate only one user for testing purposes without losing his email and / or affecting other mailboxes but there is different option for it (either from the Active Users or Exchange tab), I'm not sure how to do this-- step by step ?

Yeah got confused with the PM, the title here clearly states Hybrid :) To move a single mailbox, you can use the New-MoveRequest cmdlet: https://technet.microsoft.com/en-us/library/dd351123(v=exchg.141).aspx

Here's an example:

New-MoveRequest -Identity user@contoso.com -Remote -RemoteHostName mail.contoso.com -RemoteCredential $OnPremAdmin -TargetDeliveryDomain “Contoso.mail.onmicrosoft.com”

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Thanks a lot for this helpful information. Other than using the PowerShell command line...what will be the process to move the mailbox from Exchange 2010 to O365 ?