Convert/Connect Office 365 accounts to AD


We have this situation in Office 365:
A company decided to move all their users to Office 365. The on-premises domain is removed.
They now only have cloud users, Exchange online mailboxes and a Sharepoint Online environment.

Some major changes took place in the organization. The company fused with another company that is in lead now.
In the new situation, an Active Directory is in place and useraccounts have been made for the users, so they have now two logons: One for Office 365 and one for the active directory domain.
In this new situation, the wish of the company is to have one single login for the domain and Office 365.
"Office 365 Directory Synchronization" must become the tool to accomplish this task.

If we synchronize AD to the cloud, we will and up two accounts for every user:
One account created in the cloud, and one AD synchronized account .

I have the following questions:

Is there a way to convert "In the cloud" accounts to "Synchronized with Active Directory" accounts in Office 365 AD ?
If not,
I think we have to perform some kind of (mail) migration from the "In the Cloud" account to the "Synchronized with Active Directory" account ?  Can this be done ?
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Vasil Michev (MVP)Commented:
You have two methods of matching on-prem and cloud accounts:

1) soft-match, based on primary SMTP:
2) hard match, based on the object GUID:

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