Changing primary keys

[Carol Cheung <cacheung(at)consumercontact(dot)com>]

Hi,

The table 'house' was created with the below code (partial)

CREATE TABLE house (
  house_id bigserial unique not null,
  phone bigint not null,
  address varchar(75) not null,
  primary key(phone, address)
);

My question is how can I switch the primary key from (phone, address) to 
house_id, now that the table is created and records have been inserted.

Thanks,
C