Updating a primary key

I was surprised to see that PostgreSQL doesn't execute a multiple row update
as an atomic operation, but apparently one row at a time, with primary key
uniqueness being checked after each row update.

For example, let's say we have this table :

CREATE TABLE mytable (
pos int PRIMARY KEY,
t text );

into witch we insert two rows :

INSERT INTO mytable (pos,t) VALUES (1,'test1');
INSERT INTO mytable (pos,t) VALUES (2,'test2');

Then, in order to insert a new record in position 1, we first try this
update to bump any existing position number by 1 :

UPDATE mytable SET pos = pos + 1;

This actually raises the error "ERROR: duplicate key violates unique
constraint "mytable_pkey"".

I'd be interested in any suggestions of workaround for this.

Thanks,
Paul