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.
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)