Syntax error in DROP CONSTRAINT

Hi Forum,

I need to add a colume to existing composite primary key on a table.
I tried using following:
>> ALTER TABLE DROP CONSTRAINT tablename_pkey;
>> ALTER TABLE ADD CONSTRAINT PRIMARY KEY (col1, col2, col3, new_col4);

When I try this using Postgres 7.2.4 I get psql syntax error in DROP
CONSTRAINT.
ERROR: parser: parse error at or near ";"

E.g:
CREATE TABLE forwardinginfo
(
traptype smallint,
trapnumber smallint,
ipaddr varchar ( 16 ),
port integer DEFAULT 162,
snmpversion smallint,
PRIMARY KEY ( traptype, trapnumber, ipaddr, port )
);

ALTER TABLE forwardinginfo DROP CONSTRAINT forwardinginfo_pkey;
**** ERROR: parser: parse error at or near ";" ****

I want to add the new column after droping the existing constraint.
ALTER TABLE forwardinginfo ADD CONSTRAINT PRIMARY KEY ( traptype,
trapnumber, ipaddr, port, snmpversion);

If I try the same thing with the latest 7.5 version, I see NO error.
How do I perform this with my 7.2.4 postgresql version server?

Thanks,
Rajan.