DELETING then INSERTING record with same PK in the same TRANSACTION

Hi,

When I:

Begin a transaction

DELETE from <table> where id (PK) = <somevalue>

INSERT INTO <table> VALUES (<values - same PK>)

...

...

...

COMMIT

I get 

insert into "itinerary" ("dk", "locator") values ($1, $2) - duplicate key value violates unique constraint "itinerary_pkey"

Do I need some particular kind of settings on my transaction to be able to delete and insert afresh in the same transaction?

In case it is relevant - the first delete will lead to cascaded deletes on children.

Alternatively I wonder if I change the PK column value to "<somevalue>-FORDELETION" will I be free to insert under the same original PK value in the same transaction, then delete the FORDELETE item just before committing or will I hit the same issue?

Thanks,

Andrew