SQL & PL/SQL :: How To Change User Password In Stored Procedure

OBJECTIVE: To allow a user to change their own password after logging into an application.

BACKGROUND: I have a stored procedure where I pass the userid, old password, and new password. The stored procedure contains the following ALTER statement:

EXECUTE IMMEDIATE 'ALTER USER :uid
IDENTIFIED BY :npwd REPLACE :opwd'
USING IN v_user_id, v_new_pwd, v_old_pwd;

where v_user_id, v_new_pwd, and v_old_pwd are the arguments passed to the stored procedure.

FACTS:
(1) The procedure compiles fine;
(2) During execution, SQLCODE returns "0" (i.e. zero);
(3) My userid does have permission to change my password and execute the stored procedure;
(4) At the time I call the stored procedure, I am successfully logged into the application with my userid and old password.

PROBLEM: When I try to login with the new password I get the following error: "ORA-01017 invalid username/password; logon denied".