SBS2003 to Samba4 migration

[Daniele Dario]

Hi samba team,
in my company we have a SBS2003 DC which actually acts both as AC DC and
mail server.
Currently we are moving the mail server out of the company and next I
would remove the SBS server and use samba4 as DC.

In our topology we also have a samba3 server joined to our sbs domain
which acts as file server.

I tried setting up a samba4 (samba version 4.0.0alpha17-GIT-ccaab14)
server. I joined the server to the sbs2003 domain and all went ok
(quick-test OK, samba-tool domain join OK, samba-tool drs kcc and
showrepl OK) but if I shut down the sbs server, no user can login to the
samba3 file server.

The idea was to join samba4 to sbs2003 than remove the sbs2003 DC (I'm
not a skilled IT manager so maybe it is not a good idea). Is it wrong?

Basically I would do this staff without changing anything on the
client's side. Is it possible or do I have to provision a new domain and
than create users and groups than join the clients (winxp/vista/7) to
the newly created domain?

As a second step I would use the hw of the sbs server for the file
server.
Would it be best to set it up with samba4 (so it can also be a secondary
DC) or is it best to keep it with samba3?

Thanks in advance,
Daniele.