Trigger PLSQL on column change

BEGIN
INSERT INTO CREDIT_MASTER (CID, CDATE, CHAIN, CUSTOMER, REP, ROUTE, CRAMT)
SELECT
  H.CID,
  H.CDATE, 
  H.CHAIN, 
  H.CUSTOMER,
  H.REP,
  NULL ROUTE,
  sum(QTY*ITMPRC) CRAMT
FROM CREDIT_HEADER H, CREDIT_DETAIL D
WHERE H.CID = D.CID
  AND H.STATUS is 1
GROUP BY 
  H.CID,
  H.CDATE, 
  H.CHAIN, 
  H.CUSTOMER,
  H.REP;
  commit;
  update CREDIT_HEADER set status = 2 where status = 1 and REP is not null;
  commit;
  END;

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CREDIT_HEADER
-----------------------------
CID      NOT NULL VARCHAR2(9)   
CDATE             DATE          
CHAIN             VARCHAR2(6)   
CUSTOMER          VARCHAR2(6)   
CNAME             VARCHAR2(20)  
CCOMMENT          VARCHAR2(120) 
REP               VARCHAR2(15)  
STATUS            VARCHAR2(2)   

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CREDIT_MASTER
----------------------------
desc credit_master
Name     Null     Type         
-------- -------- ------------ 
CID      NOT NULL VARCHAR2(9)  
CDATE             DATE         
CHAIN             VARCHAR2(6)  
CUSTOMER          VARCHAR2(6)  
REP               VARCHAR2(15) 
ROUTE             VARCHAR2(20) 
CRAMT             NUMBER       

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