Downgrading an Active Directory DC
In general, there are two ways to upgrade or downgrade Samba AD DC. The first way is to replace the packages or binaries 'in-place', which may require additional steps to operate correctly. The second way is to join (or re-join) a new domain controller and optionally demoting any unused controllers. Generally speaking, newer versions should return more consistent results via replication and so this method should normally work regardless (although may be operationally trickier to do due to DNS records, naming and other topology related artifacts).
The 'in-place' method is described on the Updating Samba page. While upgrades tend to have code within the Samba daemon to auto-upgrade underlying formats and discrepancies, or rely on the Samba AD DC database check tool, downgrades of certain versions may require additional tools to run.
Downgrade compatibility (4.7 - 4.11)