gistfile1.txt

<SOME_DN>=real user DN...

add-primgroup.ldif:
############################################
dn: <SOME_DN>
changetype: modify
replace: primaryGroupID
primaryGroupID: 513
############################################

command:

# ldbmodify -H /var/db/samba4/private/sam.ldb add-primgroup.ldif
ERR: (Object class violation) "error in module samldb: Object class violation during LDB_MODIFY (65)" on DN <SOME_DN> at block before line 4
Modify failed after processing 0 records

But the user IS member of Domain Users:

# ldbedit -e vim -H /var/db/samba4/private/sam.ldb '(samaccountname=<...>)'
...
memberOf: CN=Domain Users,CN=Users,DC=...,DC=...
...