Oracle Error Message
ORA-22857: cannot modify columns of object tables
Reason for the Error
An attempt was made to alter the object table by modifing existing columns. An object table cannot be altered to modify existing columns since it is based on an object type. The table definition must be in sync with the corresponding type.
Solution
Create a new type with the desired attribute types and use it to create an object table. The new object table will have the desired columns.