Office 365 without Active Directory

Hello all,

I hope that you are well !

Well, I have a strange question, maybe you can help me.

One of my customer did the move on the cloud Office 365 for at least 500 users.

Now this customer never had an Active Directory within is organization.

So now he wants to add an Active Directory within is organization.

The question is : After configure an AD DS on premise what will happen to existing user within Office 365, I know that organization with AD DS they can use DirSync to sync their AD to Office 365, but for that particular issue what are the possibilities.

Best regards,