Hi,
The Group Policy resetting in Active Directory that you describe will have no effect on existing accounts until users try to change or update their passwords. If their passwords don't meet the password complexity requirements, they'll stay the same until
they're prompted to change them. For example, if you change the minimum password length from 6 to 8 characters, the user will not notice that until the password is changed and may not notice if they already use 8 character or longer passwords.
For more information, please refer to the article below.
http://blogs.technet.com/b/askpfeplat/archive/2013/10/11/active-directory-password-policies-when-does-a-password-policy-change-affect-a-user.aspx
Best Regards,
Jay