Change Password Email Notification On WordPress

Below is an example on how to alter the password notification email that WordPress sends users when their password changes. This simply uses the password_change_email hook and passes the $new_message_txt to it.

add_filter(
  'password_change_email',
  'wpse207879_change_password_mail_message',
  10,
  3
);
function wpse207879_change_password_mail_message(
  $pass_change_mail,
  $user,
  $userdata
) {
  $new_message_txt = __( 'Some text ###USERNAME### more text
    even more text ###EMAIL### more text after more text
    last bit of text ###SITENAME###' );
  $pass_change_mail[ 'message' ] = $new_message_txt;
  return $pass_change_mail;
}

I will likely just throw this into a Must Use plugin just so that the message is covered across the network.