The CHANGE FIELD command modifies a field element by performing
    a change in place. CDO changes the values you specify, and other
    values remain the same.

    If a field element is controlled, CDO freezes previous versions
    and allows you to modify only the highest visible version. If a
    field element is uncontrolled, CDO modifies the highest version
    unless you specify another version number.

    If a field element is controlled, you must reserve the field
    before you can issue the CHANGE FIELD command. The SHOW FIELD
    or SHOW RESERVATIONS command indicates whether this condition
    exists.

    When you change a field element that an Oracle Rdb database uses,
    you may need to integrate the database with the repository. CDO
    automatically sends a notice with the name of the database when
    this is necessary.