Oracle : ORA-28221: REPLACE not specified

You are trying to change the password of a user and getting this error.
The database is Oracle release 9.2 or higher and password complexity is enabled, either with an Oracle supplied function or with a custom function.

The REPLACE keyword was first introduced in Oracle 9.2. In previous versions of Oracle, any user could change their password using the ALTER USER ... IDENTIFIED BY ... statement.
Keep in mind that this includes ALL users regardless if they have the ALTER USER system privilege.

With Oracle 9.2 and higher, if you DO NOT have the system privilege ALTER USER and there is a PASSWORD_VERIFY_FUNCTION on a profile that is assigned to that user, then you MUST use the REPLACE keyword along with the old password.

If you have the ALTER USER system privilege, then you do not need to use REPLACE.

Example : Alter user XXX identified by replace ;

Source : http://www.dbmotive.com/oracle_error_codes.php?type=ORA&errcode=28221