Modifying Column Type in Table
Method 1 :

1. Add a Copy of the Column appending to some identifier to say its a copy i.e _CP
2. Update the New Column to the Column whose datatype is to be changed
3. Drop the Old Column
4. Rename the New Column to Old Column Name

ALTER TABLE TB1 ADD COL_CP VARCHAR2(10);

UPDATE TB1 SET COL_CP = COL1;

ALTER TABLE TB1 DROP COLUMN COL1;

ALTER TABLE TB1 RENAME COLUMN COL_CP to COL1;

In case you are upgrading from Varchar2(250) to Varchar2(4000) the above is not required

Method 2 :

1. Create a New table(TB2) with the same Columns and Type
2. Insert the Values into TB2
3. Truncate or Drop Table TB1
4. Run the Changes such as New DDL Script
5. Insert into new Table TB1 from TB2
6. Drop Table TB2

CREATE TABLE TB1(COL1 NUMBER);

INSERT INTO TB1 VALUES(10);

CREATE TABLE TB2 AS
SELECT * FROM TB1;

DROP TABLE TB1;

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New table Creation Script with New Column Types
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INSERT INTO TB1
SELECT * FROM TB2;

DROP TABLE TB2;