You may try this:
Add a new column as varchar2
alter table my_table add (new_column varchar2(1000));
UPDATE CLOB name to varchar2 column;
update my_table set new_column=dbms_lob.substr(old_column,1000,1);
After testing your data:
DROP CLOB column
alter table my_table drop column old_column
Rename varchar2 column to CLOB column name
alter table my_table rename column new_column to old_column