This is known as multiple forest and single AAD Connect configuration - so in your case you can add new AD Domain that you are rebuilding and sync it across to AAD, remove the old AD Domain from the configuration.
Sync back is only for attributes and passwords, you can setup local AD with users and groups from AAD, the reason you cannot find solution is that it is not available to rebuild the AD Domain from AAD, you should have backup and snapshot of the AD as this is proper way to restore AD.
Hope this helps.
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