ORA-30673: column to be modified is not an identity column oracle 12c

ORA-30673: column to be modified is not an identity column

Answer : - 

Currently orcle 12c does not support modifying an existing column as IDENTITY column 

Solution : - Solution is to add a new column and then drop the existing one

ALTER TABLE Tablename ADD (ColumnName_NEW NUMBER(3) GENERATED ALWAYS AS IDENTITY);