Hello @Innocent Heartvoice ,
Thank you for posting here.
Hope the information provided by LeonLaude is helpful to you.
Here is another information for your reference.
Functional levels determine the available Active Directory Domain Services (AD DS) domain or forest capabilities. They also determine which Windows Server operating systems you can run on domain controllers in the domain or forest. However, functional levels do not affect which operating systems you can run on workstations and member servers that are joined to the domain or forest.
And before you raise your forest and domain functional level to Windows 2012 R2, ensure all the Domain Controllers in the forest are 2012 R2 DCs.
Reference
Forest and Domain Functional Levels
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/active-directory-functional-levels
If anything is unclear or should you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou
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