Check Active Directory Forest and Domain Functional Level
How to read the current Active Directory forest functional level (FFL) and domain functional level (DFL), what each level unlocks, the pre-flight checklist before raising, and how to actually raise it with Set-ADForestMode / Set-ADDomainMode. Includes the schema-version-to-OS table, a decision flow for whether to raise, and the common pitfalls (forgotten DCs, Server 2019/2022 reporting Windows2016Forest, irreversibility).