Hi Nicolas,
In this scenario, you can consider using SMTP matching to let the on-premises user accounts and the existing Office 365 user accounts match. For detailed information, you can refer to the following KB article:
How to use SMTP matching to match on-premises user accounts to Office 365 user accounts for directory synchronization
http://support.microsoft.com/kb/2641663
Doing SMTP match will not cause data lose.
Thanks,
Mindy Pan