ORA-22858: invalid alteration of datatype (convert varchar2 to clob)

When you convert a field from varchar2 to clob
raise an error ORA-22858: invalid alteration of datatype

SQL> create table vty_ra1 (a varchar2(10));



SQL> alter table vty_ra1 modify a clob;



ORA-22858: invalid alteration of datatype

workaround:

alter table vty_ra1 modify a long;




alter table vty_ra1 modify a clob;