A cloud-based identity and access management service for securing user authentication and resource access
Hi, first of, you do not need the password for the sync account in Azure AD to change your sync scope. The only thing you need is to re-run the wizard and change your scope settings again in the wizard. That is the easiest way to change this. If you go in through the sync enginge, you can actually use any account with permissions in AD to change the settings, it will not replace the service account actually being used for sync. In fact if you allowed AAD Connect to create the service account on your behalf, you would never know the password of this account at all.
Secondly, you must be aware that there is a fail-safe implemented in AAD Connect (prevent accidential delete) . So if the scope change deletes more than 500 items, it will halt and you need to go into powershell to temporariliy disable the failsafe using the cmdlet Enable-ADSyncExportDeletionThreshold
Hope this helps.