Azure Entra Connect

Hi all I am looking for some direction on how to deal with an issue that has arisen.

I have a client with an in-house on premises domain that has 2 domain controllers.

1 DC is Windows 2012 R2 and has an older version of Active Sync installed that stopped syncing in October 2023. The other DC is Windows 2022 and will become the 'PDC" soon as the old one will be retired soon.  Due to many reasons this was left past the desired time to be dealt with.

I have looked at this until my eyes have crossed.

Should I just disable sync using the web based Powershell and then start new with Entra Connect on the Windows 2022 server? The sync was mostly for efficiency so that the exchange O365 match users as we created them with password sync as well. There fore there will not be any issues I can see considering the sync has not been active since October 2023?

Or, should I setup the new server in staging mode, test the settings and then make it active? How will the orphaned server be dealt with since it has no way to even communcate that it is in staging mode?

Thanks for any thoughts