ID
---
0
1
2
3
..
..
100

ID
---
1      Eg :(  0 +1)
2          (  1 +1) 
3          (  2 +1)
4          (  3 +1) 
..      ....
..      ... 

SQL> update employees set employee_id = employee_id +1;
update employees set employee_id = employee_id +1
*
ERROR at line 1:
ORA-02292: integrity constraint (HR.DEPT_MGR_FK) violated - child record found

Begin
For i in Reverse 1..n Loop
Update Emp set Col1=i+1 where Col1=i;
End loop;
Commit;
End;

1.Disable the PK constraint on the column.
2. Update Emp set Col1=Col1+1;
3. Then Enable the constraint...