SQL & PL/SQL :: Date To Number Conversion

I want to change a table datatype from date to number where already existing data should get convert.any possibility of doing where i tried like this but no get changing. Even as Julian format is working a bit i want the data to come as GMT format

Scenarios where i tried are like this.

ALTER TABLE Contacts ADD ALERT_DATE1 NUMBER(20,0)
/
UPDATE Contacts SET ALERT_DATE1 = TO_NUMBER(ALERT_DATE)
/
ALTER TABLE Contacts DROP COLUMN ALERT_DATE
/
ALTER TABLE Contacts RENAME COLUMN ALERT_DATE1 TO ALERT_DATE
/

but second statement failing.

Julian fomat like

SELECT sysdate,
TO_CHAR(sysdate, 'J'),
TO_DATE(TO_CHAR(sysdate, 'J'),'J')
FROM dual;