Hello,
Thank you for posting in Q&A forum.
Unfortunately, once DFL and FFL are raised, they cannot be downgraded without forest restoration. This is because raising DFL and FFL makes irreversible changes to the Active Directory database schema.
It is recommended that you test the upgrade in a non-production environment before applying it to your production environment. This allows problems that may occur to be identified and resolved without impacting your production environment.
If you decide to proceed with the upgrade, make sure you have a complete backup of your Active Directory environment. If something goes wrong you can restore the backup, effectively "downgrading" your DFL and FFL.
For more information, please refer to the link:
Active Directory Domain Services Functional Levels in Windows Server | Microsoft Learn
AD Forest Recovery - Backing up a full server | Microsoft Learn
I hope the information above is helpful.
Best Regards,
Yanhong Liu
============================================
If the Answer is helpful, please click "Accept Answer" and upvote it.