Deploy Hybrid existing Office 365 with new AD and new Exchange server

Hi Md. Shamim Reza,

I think best option could be:

1. Sync Users with ADConnect
2. Enable Exchange Hybrid Configuration (I think this configuration could be optional, based on your needs)
3. For users that already have an Office 365 Mailbox, don't create a mailbox but use powershell command Enable-RemoteMailbox (https://docs.microsoft.com/it-it/powershell/module/exchange/enable-remotemailbox?view=exchange-ps). In this way users should make a soft match (via UPN and ProxyAddress) with those who have mailboxes already present on Exchange Online.

My advice is if possible to replicate the scenario in a laboratory and test it before going into production.

Best regards