In order to convert a column datatype from varchar2 to clob do:

step 1 - add a column with clob datatype:

_alter table add( clob);

_

step 2 - copy data from the source column to the new one:

_update set = ;

_

step 3 - rename the new column created in the step 1 in order to complete the process:

alter table__rename column to ;
alter table__rename column to ;