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Short version: data-preserving downgrades between Samba versions are not supported
With Samba development there are cases when on-disk database format evolves.
In general, Samba Team attempts to maintain forward compatibility and
automatically upgrade databases during runtime when requires.
However, when downgrade is required Samba will not perform downgrade to
existing databases. It may be impossible if new features that caused database
upgrade are in use. Thus, one needs to consider a downgrade procedure before
actually downgrading Samba setup.
Please always perform back up prior both upgrading and downgrading across major
version changes. Restoring database files is easiest and simplest way to get to
previously working setup.
Easiest way to downgrade is to remove all created databases and start from scratch.
This means losing all authentication and domain relationship data, as well as
user databases (in case of tdb storage), printers, registry settings, and winbindd
Remove databases in following locations:
In particular, registry settings are known to prevent running downgraded versions
(Samba 4 to Samba 3) as registry format has changed between Samba 3 and Samba 4.