Thank you for posting here.
According to our description, we would like to know whether there is any impact if we raise the functional level from Windows 2008 R2 to Windows 2016.
Once the Functional Level has been upgraded, new DCs on running on downlevel versions of Windows Server cannot be added to the domain or forest. To prevent these issues from arising, a new DC must be at the same level, or greater, than the functional level of the domain or forest.
The second restriction, for which there is a limited exception on Windows Server 2008 R2, is that once upgraded, the Domain or Forest Functional Level cannot later be downgraded.
The process of raising DFL/FFL is not a reversible operation without restore. Before proceeding further with the change, please review our AD environment. Make sure that our AD environment is healthy.
We can set the domain functional level to a value that is equal to or higher than the forest functional level. Also, we will need to have DCs that are running OS with the same level as DFL or higher.
For more information, please refer to:
Understanding Active Directory Domain Services (AD DS) Functional Levels
What is the Impact of Upgrading the Domain or Forest Functional Level?
Hope the information is helpful. Please let us know if we would like further assistance.