SQL> create table test
  2  (id number primary key,
  3   name varchar2(20));

Table created.

SQL> insert into test (id, name) values (1, 'Littlefoot');

1 row created.

SQL> insert into test (id, name) values (2, 'Bigfoot');

1 row created.

SQL> update test set
  2    id = 1
  3    where id = 2;
update test set
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.SYS_C0071331) violated


SQL> update test set
  2    id = 3
  3    where id = 2;

1 row updated.

SQL> update test set
  2    name = 'Check it once again!'        --> Do it!
  3    where id = 1;

1 row updated.

SQL>