How To Change Password Hash Algorithm From MD5 to SHA-256

I am trying to get Postgres to use SHA as the password hash algorithm instead of MD5. This is a security requirement. I am running PostgreSQL 8.4 on a Red Hat Linux platform. I have installed pgcrypto and ran pgcrypto.sql. I also created a function SHA-256. The function looks like:

CREATE OR REPLACE FUNCTION sha256(bytea) returns text AS $$
SELECT encode(digest($1, 'sha256'), 'hex')
$$ LANGUAGE SQL STRICT IMMUTABLE;

I then created a new user. Then I queried pg_shadow and the encrypted password still has md5 at the beginning of the hash. How do I get postgres to use SHA-256?

Any help would be greatly appreciated.

Ericka Romain