Hi,

Customer has working tenant with data inside and you need to convert it from Cloud-Only to Synced.

It can be done with the following choices:

UPN-matcing

https://support.microsoft.com/en-us/help/3164442/how-to-use-upn-matching-for-identity-synchronization-in-office-365,-azure,-or-intune

    SMTP-matching

    http://www.ivchenko.pro/Blog/Post/23/Merging-on-premises-and-Office-365-users

    HARD-matching

    https://dirteam.com/dave/2014/08/15/fixing-office-365-dirsync-account-matching-issues/

    Or by using little bit more effort and exporting user attributes from Office 365 to csv-file.

    https://365lab.net/2014/04/18/office-365-migrate-from-cloud-identities-to-dirsync/

    Johan, the Author of TailSpinToys has wrote the script and here are the attributes it exports.

    GivenName
    sn (SurName)
    DisplayName
    mail
    proxyAddresses

    Excellent, just excellent 🙂