samba - Feb 2008 - cannot logon domain after upgrade 3.0.20 to 3.0.28

I have a working copy of Samba 3.0.20 (installed under /usr/samba) configured as
a PDC.
Recently I install Samba 3.0.28 under /usr/newsamba, and copy the files
lib/smb.conf (with path modifications), private/*, var/locks/* to the new
locations.
Following the HOWTO, the groups "Domain Admins", "Domain
Users", and "Domain Guests" (with RID's 512, 513, 514 resp.)
are added.
Then I fire up "nmbd -D" and "smbd -D". The command
"net getlocalsid" reports the correct server SID. Workgroup access is
OK, but domain logon fails. The log file (level 2) reads :

[2008/01/24 08:26:24, 1] lib/util_sock.c:get_peer_name(1191)
  Gethostbyaddr failed for xxx.xxx.xxx.xxx
[2008/01/24 08:26:24, 2] lib/access.c:check_access(323)
  Allowed connection from xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx)
[2008/01/24 08:26:24, 2] smbd/reply.c:reply_special(324)
  netbios connect: name1=SMBSERVER            name2=SMBCLIENT
[2008/01/24 08:26:24, 2] smbd/reply.c:reply_special(331)
  netbios connect: local=smbserver remote=smbclient, name type = 0
[2008/01/24 08:26:24, 2] smbd/sesssetup.c:setup_new_vc_session(1200)
  setup_new_vc_session: New VC == 0, if NT4.x compatible we would close all old
resources.
[2008/01/24 08:26:25, 2] smbd/sesssetup.c:setup_new_vc_session(1200)
  setup_new_vc_session: New VC == 0, if NT4.x compatible we would close all old
resources.
[2008/01/24 08:26:25, 2] lib/access.c:check_access(323)
  Allowed connection from xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx)
[2008/01/24 08:26:25, 2] libsmb/credentials.c:creds_server_check(220)
  creds_server_check: credentials check failed.
[2008/01/24 08:26:25, 0] rpc_server/srv_netlog_nt.c:_net_auth_2(478)
  _net_auth2: creds_server_check failed. Rejecting auth request from client
SMBCLIENT machine account SMBCLIENT$

Although this can be fixed by re-joining the domain for each workstation, but
it's not sound to do so for each upgrade.

Could someone tell me what I've done wrong in the migration ?
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