[Samba] Impossible to change Primary group

[drweb]

I have Samba PDC and use tdbsam backend. I use the version 3.0.11, but 
recently I have exchanged it for the version 3.0.23c. Now I have a 
number of problem. Here is one of then.
I wish to move one user "gad" from Domain Users (RID 513) to Domain 
Guests (RID 514).

Now Primary group of user "gad" is "Domain Users":
pdbedit -Lv gad -d0
Unix username:        gad
NT username:
Account Flags:        [U          ]
User SID:             S-1-5-21-3738234612-4042114426-1695072666-3024
Primary Group SID:    S-1-5-21-3738234612-4042114426-1695072666-513
Full Name:            Гад еще тот
Home Directory:       \\unix\gad
HomeDir Drive:        N:
Logon Script:         scripts\start.bat
Profile Path:         \\unix\profiles\gad
Domain:               ACC
Account desc:
Workstations:
Munged dial:
Logon time:           0
Logoff time:          Tue, 19 Jan 2038 06:14:07 MSK
Kickoff time:         Tue, 19 Jan 2038 06:14:07 MSK
Password last set:    Sat, 25 Nov 2006 16:24:33 MSK
Password can change:  Sat, 25 Nov 2006 16:24:33 MSK
Password must change: Tue, 19 Jan 2038 06:14:07 MSK
Last bad password   : 0
Bad password count  : 0
Logon hours         : FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF


By command:
pdbedit -r gad -G 514 -d0
I cann't change primary group.

By command:
pdbedit -r gad -G 514 -d5
I cann't change primary group too :) but I find out an error (text of my 
smb.conf is skipped):
Attempting to register passdb backend smbpasswd
Successfully added passdb backend 'smbpasswd'
Attempting to register passdb backend tdbsam
Successfully added passdb backend 'tdbsam'
Attempting to find an passdb backend to match tdbsam (tdbsam)
Found pdb backend tdbsam
pdb backend tdbsam has a valid init
Netbios name list:-
my_netbios_names[0]="UNIX"
Attempting to find an passdb backend to match tdbsam (tdbsam)
Found pdb backend tdbsam
pdb backend tdbsam has a valid init
tdbsam_open: successfully opened /usr/local/etc/samba/passdb.tdb
lookup_global_sam_rid: looking up RID 513.
pdb_getsampwrid (TDB): error looking up RID 513 by key RID_00000201.
  Error: Record does not exist
store_gid_sid_cache: gid 1069 in cache -> 
S-1-5-21-3738234612-4042114426-1695072666-513
Unix username:        gad
NT username:
Account Flags:        [U          ]
User SID:             S-1-5-21-3738234612-4042114426-1695072666-3024
Primary Group SID:    S-1-5-21-3738234612-4042114426-1695072666-513



What may be wrong?