How to switch to a type of field varchar2 clob

Specific operation is as follows:

Bookdetails original field is varchar2 (4000), the original table has traffic data

ALTER TABLE test
ADD (bookdetails01 CLOB);

UPDATE test SET bookdetails01=bookdetails;
COMMIT;

ALTER TABLE test DROP COLUMN bookdetails;

ALTER TABLE atable
RENAME COLUMN bookdetails01 TO bookdetails;