[Samba] Best strategy to move/upgrade Samba 3 PDC to new Samba 4 server

[MI]

And of course, the important related question: Keep it an NT4 domain, or upgrade to AD?

We have a Samba 3.5.6 PDC with OpenLDAP, serving about 20 machines/users. (Debian 6 
"Squeeze")
OpenLDAP is also used for many other services: Unix user accounts and groups, Mail 
(Postfix/Dovecot), wiki, PostgreSQL, Calendar server, FTP, Apache, ...

The new machine is Debian 8 "Jessie", and has Samba 4.1.17.

- Classic upgrade to AD?

A lot of documentation concentrates on AD. But I'm not sure what benefit I would get 
from moving to AD. All we really need is file sharing to Windows (and a few 
Mac/Linux) machines, and running netlogon scripts for the Windows machines.

Is the complexity of AD worth the trouble? What happens to all the other services 
currently using OpenLDAP? I guess we would also need to configure our Bind 9 DNS to 
accommodate the needs of AD.

- Or NT4 BDC to PDC?

It seems much simpler and sufficient for our needs to configure the new server as a 
plain NT4 BDC, then promote it to PDC and remove the old server.

However, I have not done that before, and there doesn't seem to be much recent 
documentation about this. Is there so little documentation because it's so simple and 
doesn't need much help? Or is just nobody doing it this way? And if the latter, why 
not? What are the pitfalls to expect?


Thanks,