At home and at work I'm using Samba4 as domain cotroller to provide LDAP authentication to all services, so users have no need to remember all these logins and passwords. It was working flawlessly until I've upgraded my Debian machine which serves as domain controller from Jessie to Stretch.
While upgrading Debian 8 to 9 apt throw away error about inability to upgrade samba packages. Why? Well, it was easy enough - post-installation script tries to restart smbd and nmbd services with systemd which isn't what we want, because Debian have samba-ad-dc unit for controlling Samba in AD DC mode. First two units can't actually coexists (and colaunch, corestart, etc.) with last one because first two controls two (more or less) independent systems, while last one controls both of them.
Solution is to mask smbd and nmbd units before upgrading:
systemctl mask smbd
systemctl mask nmbd
Even if you encounter this problem after upgrading - don't worry, mask these units and continue with upgrade.
By the way, this recipe also applies to simple samba upgrade.