The newest domain functional levels and the newest forest functional levels affect only the way that the domain controllers operate together as a group. The clients that interact with the domain or with the forest are unaffected. Additionally, applications are unaffected by changes to the domain functional levels or to the forest functional levels. However, applications can take advantage of the newest domain features and of the newest forest features.
1. Check replication status and AD health.
dcdiag /e /test:sysvolcheck /test:advertising
netdom.exe query fsmo
2. Raise the Domain Functional Level.
Set-ADDomainMode -Identity local.example Windows2016Domain
3. Check it.
repadmin.exe /showattr *.local.example "dc=local,dc=example" /atts:msDS-Behavior-Version
4. Raise the Forest Functional Level.
Set-ADForestMode -Identity local.example -ForestMode Windows2016Forest