@Patrick M
If you are looking for completely decommission on-premises AD then you will have to break the link between on-premises and Azure AD. AD connect will not be functional to sync objects from Azure AD to cloud.
To stop the sync you can either stop the AD connect service on the on-premises server or you can uninstall AD connect from the server.
Post this you will still not be able to manage previously synced user objects in Azure AD. To manage these users you will have to follow below steps,
- Open Windows PowerShell as administrator.
- Run command "Install-module msonline"
- Connect-Msolservice
- Above command will prompt for credentials. You can enter global admin credentials in the prompt.
- Post this you can run command Set-MsolDirSyncEnabled -EnableDirsync $False
- Above command will change all users as "cloud only" users. This will let you manage user objects in Azure AD itself.
Post this you can decommission your on-premises environment.
You can also refer below article,
https://learn.microsoft.com/en-us/microsoft-365/enterprise/turn-off-directory-synchronization?view=o365-worldwide
Let me know if you have any further questions.
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