Open your power shell and type
Get-MsolDirSyncFeatures -Feature EnableSoftMatchOnUpn
Soft match means you get an bidirectional snych. If this feature is not active within you setup you need to activate it first with
Set-MsolDirSyncFeature -Feature EnableSoftMatchOnUpn -Enable $true
After this a forced delta synch should deliver the wanted results in the hybrid config.
IF you have colliding objects afterwards, you could take a look into here.
https://support.microsoft.com/en-us/help/2523192/user-names-in-office-365-azure-or-intune-don-t-match-the-on-premises-u
All Accounts could be synched and combined again without any problems. There is also a error correction in your azure active directory admin interface.