It surprises me how much I run across this one but then again I have been guilty of it as well.
There is a very important first step that I find skipped over and forgotten quite often when it comes to running an Exchange migration. Or really any other kind of migration. Have you taken a system state backup of AD yet? No? Then you’re just spinning the bottle and hoping it doesn’t end up with you getting cozy with Microsoft’s support hoping that they can fixed your screwed up Active Directory.
Don’t make the mistake of assuming backups are working
I made this mistake once upon a time. It was from one of the first Exchange migrations I was running. I didn’t feel like being bothered to take a backup of AD as the server was a really slow server. I was confident that the nightly backups had taken care of everything anyhow. Though I didn’t bother to validate this. So I went directly into running the migration and everything was going smoothly at first. Everything looked like it was running great. But then part of the way through I found that AD replication had broken and that it possibly had been that way for a while. It would have been easy to roll back to an AD backup, correct the problem and then retrace my steps but unfortunately that wasn’t an option. Because I hadn’t taken a backup. The nightly backups hadn’t worked in several months either. That lead into a call with Microsoft later on and then having to spend even more hours fixing things manually via ADSIedit when they couldn’t figure it out.
I don’t want to be the one cleaning up after you
It is a very simple step to take at the very beginning. Just grab a backup before you run your first setup.com /PrepareAD. While you’re at it, why don’t you test the backup of your current mail server and make sure that it is working ok as well. Trust me on this. You don’t want to be the guy to explain to your boss that the data is gone as your only valid backup is from 3 years ago. Your backups are working, right? You might want to double check on that just to be sure. I recommend a mock restore for that extra bit of assurance.