create table reg_user
(
USERNAME VARCHAR2(50),
PASSWORD VARCHAR2(20)
)
***
insert all
into reg_user (USERNAME,PASSWORD) values ('A','aa')
into reg_user (USERNAME,PASSWORD) values ('B','vv')
into reg_user (USERNAME,PASSWORD) values ('C','ff')
into reg_user (USERNAME,PASSWORD) values ('D','ee') 
--hundreds of ROWS inserted
select * from dual;

alter table reg_user add userid number;

update reg_user set 
  userid = rownum;

 UPDATE reg_user SET PASSWORD='zzz' WHERE rowid=(
SELECT rowid FROM reg_user WHERE rownum <=3 
MINUS
SELECT rowid FROM reg_user
 WHERE rownum <3
 );

create table t1
(
USERNAME VARCHAR2(50),
PASSWORD VARCHAR2(20)
)
***
insert all
into t1(USERNAME,PASSWORD) values ('A','aa')
into t1(USERNAME,PASSWORD) values ('B1','vv')
into t1(USERNAME,PASSWORD) values ('C1','iiii')
into t1(USERNAME,PASSWORD) values ('D1','ee') 
--other rows
select * from dual;

DECLARE   
  CURSOR c1 IS
   SELECT username FROM reg_user
    FOR update;
  init NUMBER(10);
  lRow NUMBER(10);

begin 
   SELECT Max(userid) into init FROM reg_user;
   SELECT Count(*) INTO lRow FROM reg_user; 
   init:= init+1;
 -- Dbms_Output.put_line(init);
 -- Dbms_Output.put_line(lRow);  

 FOR i IN init..lRow LOOP
 UPDATE reg_user SET id = i WHERE CURRENT OF c1;
 END LOOP;     
END;
*******************
ORA-01001: invalid cursor
ORA-06512:

DECLARE 
uname varchar2(15);  
  CURSOR c1 IS
   SELECT username FROM reg_user
    FOR update;
  init NUMBER(10);
  lRow NUMBER(10);   
BEGIN 
   SELECT Max(id) into init FROM reg_user;
   SELECT Count(*) INTO lRow FROM reg_user; 
   init:= init+1;
 -- Dbms_Output.put_line(init);
 -- Dbms_Output.put_line(lRow); 
 OPEN c1;
 LOOP
 FETCH c1 INTO uname;
 EXIT WHEN (c1%notfound); 
 FOR i IN init..lRow LOOP  
 UPDATE reg_user SET id = i WHERE CURRENT OF c1;
  END LOOP;
 END loop;    
 CLOSE c1;   
END;

orcl> update scott.emp set sal=rownum;

14 rows updated.

orcl> select ename,sal from emp;

ENAME             SAL
---------- ----------
SMITH               1
ALLEN               2
WARD                3
JONES               4
MARTIN              5
BLAKE               6
CLARK               7
SCOTT               8
KING                9
TURNER             10
ADAMS              11
JAMES              12
FORD               13
MILLER             14

14 rows selected.

orcl>

SQL> create table reg_user as
  2    select ename from emp;

Table created.

SQL> alter table reg_user add userid number;

Table altered.

SQL> declare
  2    cursor cur_e is
  3      select ename from reg_user
  4      for update of userid;
  5
  6    i number := 1;
  7
  8  begin
  9    for cur_r in cur_e loop
 10      update reg_user set
 11        userid = i
 12        where current of cur_e;
 13
 14      i := i + 1;
 15    end loop;
 16  end;
 17  /

PL/SQL procedure successfully completed.

SQL> select * from reg_user
  2  order by userid;

ENAME          USERID
---------- ----------
SMITH               1
ALLEN               2
WARD                3
JONES               4
MARTIN              5
BLAKE               6
CLARK               7
SCOTT               8
KING                9
TURNER             10
ADAMS              11
JAMES              12
FORD               13
MILLER             14

14 rows selected.

SQL>