Hi @Arief Hardiansyah ,
the answer to this particulaar question is clearly fromulated by Ned Pyle (obne of Microsoft's greatest AD Experts) here:
What is the Impact of Upgrading the Domain or Forest Functional Level?
The summary is what gives a clear answer to your question. From the article:
To summarize, the Domain or Forest Functional Levels are flags that tell Active Directory and other Windows components that all DCs in the domain or forest are at a certain minimal level. When that occurs, new features that require a minimum OS on all DCs are enabled and can be leveraged by the Administrator. Older functionality is still supported so any applications or services that used those functions will continue to work as before -- queries will be answered, domain or forest trusts will still be valid, and all should remain right with the world.
Still, please considering this part also, as chnages in AD can sometimes be hard to revert:
Even after all this, however, there is a great concern about the change being irreversible, so that you must have a rollback plan just in case something unforeseen and catastrophic occurs to Active Directory. This is another common question, and there is a supported mechanism to restore the Domain or Forest Functional Level. You take a System State back up of one DC in each domain in the forest. To recover, flatten all the DCs in the forest, restore one for each domain from the backup, and then DCPROMO the rest back into their respective domains. This is a Forest Restore, and the steps are outlined in detail in the following guide:
Planning for Active Directory Forest Recovery
Hope I could help you out!
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