using ALTER TABLE in ORACLE 9i to RENAME NOT NULL CONSTRAINTS IN A LOOP

I am getting an error message in the following SQL at the two lines where I invoke ALTER.  What I need to do is rename all NOT NULL constraints in my database to a more meaningful name.  Can someone help me with this?  I need to get this done quick!

DROP TABLE TEMP_CONSTRAINT_RENAMER;
CREATE TABLE TEMP_CONSTRAINT_RENAMER (
aTABLE_NAME VARCHAR2(32),
aCOLUMN_NAME VARCHAR2(32),
aOLD_CONSTRAINT_NAME VARCHAR2(32),
aNEW_CONSTRAINT_NAME VARCHAR2(32)
);

INSERT INTO TEMP_CONSTRAINT_RENAMER (aTABLE_NAME,aCOLUMN_NAME,aOLD_CONSTRAINT_NAME,aNEW_CONSTRAINT_NAME)
SELECT TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME, 'NN_' || substr(TABLE_NAME,6,12) || '_' || substr(COLUMN_NAME,1,12) || '_' ||  ROWNUM AS NEW_CONSTRAINT_NAME
FROM USER_CONS_COLUMNS
WHERE CONSTRAINT_NAME LIKE 'SYS%';

CREATE OR REPLACE PROCEDURE TEMP_MAKE_NN_NAMES IS

CURSOR C1 IS SELECT ATABLE_NAME,ACOLUMN_NAME,AOLD_CONSTRAINT_NAME,ANEW_CONSTRAINT_NAME FROM TEMP_CONSTRAINT_RENAMER;

myTABLE_NAME VARCHAR(32);
myCOLUMN_NAME VARCHAR(32);
myOLD_CONSTRAINT_NAME VARCHAR(32);
myNEW_CONSTRAINT_NAME VARCHAR(32);

BEGIN
  FETCH C1 INTO myTABLE_NAME, myCOLUMN_NAME, myOLD_CONSTRAINT_NAME, myNEW_CONSTRAINT_NAME;
  WHILE C1%FOUND LOOP
    /* THE FOLLOWING TWO LINES OF CODE WILL NOT COMPILE!  HELP!
     ALTER TABLE myTABLE_NAME DROP CONSTRAINT myOLD_CONSTRAINT_NAME;
     ALTER TABLE myTABLE_NAME MODIFY myCOLUMN_NAME CONSTRAINT myNEW_CONSTRAINT_NAME NOT NULL;
     FETCH C1 INTO myTABLE_NAME, myCOLUMN_NAME, myOLD_CONSTRAINT_NAME, myNEW_CONSTRAINT_NAME;
  END LOOP;
  CLOSE C1;
END TEMP_MAKE_NN_NAMES;

/* EXEC TEMP_MAKE_NN_NAMES;