Hello,
Unfortunately, there is no option to upgrade a functional level unless the PDC (all DCs recommended) is upgraded to at least the same OS version as the DFL. In-Place upgrade is never recommended, as the recommendation from Microsoft is to promote a new DC, migrate all the roles into it, then raise functional level.
You can read more about the process and supportability at the official article: https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/upgrade-domain-controllers
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