Several months ago, I migrated our corporate email from our onprem exchange 2k7 server to office 365. Everything was successful and our email is working. We are using AD sync to sync our UN and PW from our domain (which this works well). We want to decommission our old exchange server and turn it down completely. I am reading mixed answers if this is possible. Some say we must have an on prem exchange to manage the users and boxes, and others say no. My question is, can we decommission our exchange server while keeping AD sync from our domain server and be able to manage everything from the cloud?
You can. But you will be in unsupported configuration. Since you are using DirSync, you need to manage the objects On-Prem, and the only *supported* method of doing so when it comes to Exchange objects are the Exchange management tools.
If you don't care about the "supported" bit, you can just as well manage them via ADUC/PowerShell/whatever.
A bit of a late reply, but hopefully this helps. The Microsoft best
practice method of decommissioning Exchange is to uninstall it. Doing so
will wipe all the email and Exchange related attributes in AD. By using
DirSync/Azure AD Connect you need to pause the sync before uninstalled,
and then filling all the email attributes again. Or, turn off the
Exchange server and using ADSIEdit to manually remove all containers and
objects associated with Exchange. Ran in to a scenario where we followed
the technet article provided by an Microsoft Support Engineer to
decommission the on-prem Exchange server, and when DirSync ran after it
had disconnected all the mailboxes in the cloud because the proxyAddress
and mail attributes were cleared on all AD objects.