Fastest way to change column type from varchar2 to number on large table in Oracle

I have a table which contains a VARCHAR2 column that must me changed into NUMBER.

The table contains more than 30 millions rows.

The table definition :

CREATE TABLE EFF (
    MOD         VARCHAR2(6)     NOT NULL,
    MP          VARCHAR2(6)     NOT NULL,
    PROG        VARCHAR2(1)     NOT NULL,
    RNK         VARCHAR2(4)     NOT NULL,
    EFF_STS     VARCHAR2(1),
    EFF_PART    VARCHAR2(1),
    CONSTRAINT PK_EFF PRIMARY KEY (MOD, MP, PROG, RNK)
);

The code I used on Oracle 11g to change the RNK column type :

ALTER TABLE EFF ADD (NRNK NUMBER(5));
UPDATE EFF SET NRNK = TO_NUMBER(RNK);
ALTER TABLE EFF MODIFY (NRNK NUMBER(5) NOT NULL);

Then I would modify the primary key and drop the old column :

ALTER TABLE EFF DROP PRIMARY KEY DROP INDEX;
ALTER TABLE EFF ADD CONSTRAINT PK_EFF PRIMARY KEY (MOD, MP, PROG, NRNK);
ALTER TABLE EFF DROP COLUMN RNK;

The UPDATE phase is very long, at least 20 minutes.
What could be the best way to do it faster ?

There is no indexes other than the primary key.