Follow these steps to demote a WS2016 DC to a 2012r2 domain so 2012r2 DC’s can join the domain.
Open Admin PS on the domain controller and type:
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(Get-ADForest).ForestMode Set-ADForestMode -ForestMode Windows2012r2Forest (Get-ADForest).ForestMode (Get-ADDomain).DomainMode Set-ADDomainMode -DomainMode Windows2012r2Domain (Get-ADDomain).DomainMode
You really only need the two Set- commands, I just put the others to see the before and after – look at the properties before and after you change them!