Postgres unique index checking and atomic transactions

So I have to adjust a primary key by adding one to every existing record.
Obviously this isn't a routine operation, my data model isn't that messed up.
It's a one-time manual operation.

However when I tried to do the equivalent of:

  update tab set pk = pk + 1

I got

  ERROR:  Cannot insert a duplicate key into unique index tab_pkey

Is that right? Obviously after completing the query there would be no
duplicate keys. Is this a case where I would need deferred constraints to
allow this? Even for immediate constraints shouldn't a single sql update be
able to go ahead as long as it leaves things in a consistent state?

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greg