Oracle Applications by default restrict you to reset the old earlier used password. The reason being all OLD passwords are stored in FND_USER_PREFERENCES table. We can use the following queries to see all the entries (though all passwords will be in encrypted format)

select *
from applsys.fnd_user_preferences
where user_name = 'SYSADMIN'
and module_name = 'PWDVALREUSE'

In order to reset/reuse the earlier USED password, one can delete all the entries from above mentioned table

DELETE
from applsys.fnd_user_preferences
where user_name = 'SYSADMIN'
and module_name = 'PWDVALREUSE';

COMMIT;