ID INFO
---------- ----------
     2 a
     1 b
     3 c
     4 d
     5 e

update t1 set id= 2
	where id = 1;

update t1 set id = 1
	where id = 2;

......

SQL> update t1
 2 set id = case when id = 1
 3 then 2
 4 when id = 2
 5 then 1
 6 else id end;

5 rows updated.

SQL> select * from t1;

    ID INFO
---------- ----------
     2 a
     1 b
     3 c
     4 d
     5 e

bill@bill=>update t2
set id = case when id = 1
then 2
when id = 2
then 1
else id end;
ERROR: duplicate key value violates unique constraint "t2_pkey"
DETAIL: Key (id)=(2) already exists.