Hello,
In the case you didn’t know, since Windows Server 2008, you can rollback your Domain Functional Level in certain conditions, if you look at this link, you’ll get all the details :
Here a recapitulation :
And if you activated the “RecycleBin” :
Note : The rollback can only be done with PowerShell “Set-ADDomainMode” cmdlet from the ActiveDirectory PowerShell module.