APPS DB user password changed or user locked after database upgrade from 11.1.0.7 to 12.1.0.2 (EBS v

After upgrading the EBS DB from 11.1.0.7 to 12.1.0.2, the customer can not log in to the DB with the apps user with the original password any more, they get invalid password error, they should not change the apps passwd from SQLPLUS.

How can we get the apps password changed correctly?

Why did the password get changed during the db upgrade?

They have not done any post-upgrade steps, so they can not use FNDCPASS utility to change it.