Hi Michael,
Provided that you have setup DirSync in your O365 environment to use the objectGUID from Active Directory, this is a simple task that should be completed within 1-2 hours max using Powershell. If you're unsure or you possibly used a different identifier when importing the user accounts, it is still possible but the difficulty of the task may be skewed.
Let me know if you have any questions.
Regards,
Eli