2-Way Azure Active Directory Initial Sync

Hello All,

I have a client who unfortunately lost their  DC in a disaster and did not have a viable backup. Their DC was synced to Azure AD. I have started the process of rebuilding the domain but I have not yet reconfigured the AAD sync. I was wondering if, rather than manually recreating every account and group on-prem, if I turn on the sync will it do a two way sync and create those accounts and groups on my on-prem DC? Or, would it delete the existing accounts and groups in Azure? I am nervous to reenable the sync for fear of losing mailboxes etc.

Thanks in advance!