Could the administrators post a simple handout regarding the upcoming login changes. A new password intimates the old one will no longer work. This is especially relevant to the people who will NOT want anything to do with a login via a social media account.
It would be nice to have the transition explained before it actually happens online. This, of course, assumes that the changes will not be optional.
Hope the teenagers will be thrilled as this seems to be mostly directed at the next generations. Facebook and Avatars! 