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
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 🙂