Defining password complexity and its action on "grace" state USERS
The password complexity is managed by a function. You can edit the function to fit your requirements or disable it at all. 

Check this link : https://docs.oracle.com/en/database/oracle/oracle-database/18/dbseg/configuring-authentication.html#GUID-A39E191B-4A06-442D-94C7-5882B73DDCFA
On the section "Managing the Complexity of Passwords"

For the password "grace" time, you should check the Profiles and modify them accordingly. 
On the same link check for "PASSWORD_GRACE_TIME"

If you want, you can disable the password verify function, assuming the all your users are in the default profile, alter profile default limit PASSWORD_VERIFY_FUNCTION NULL;