The key change from the Spanish Grand Prix arrangement is the removal of a winglet that Ferrari claimed served a structural role in reducing vibration of the mirrors but which the FIA felt was being exploited primarily for aerodynamic gain.
It remains to be seen whether rival teams will follow the Scuderia's lead in adopting this approach.
Technical analysis from Mark Hughes...
The winglet (indicated by a red arrow in the drawing below) used in conjunction with the mirrors in Spain was connected to the mirror by a thin cable which the team claimed enabled the mirror and winglet to be considered as one halo-mounted piece, with the winglet serving a structural role in reducing vibration of the mirrors.
The FIA countered that if this were the case, the winglet could have been mounted much lower (but where its aerodynamic effect in turning the upward wake of the airflow back down would have been much less) and from the same mounting point as the mirror. They concluded therefore, that the second mounting point implied that the primary purpose of the winglet was not structural but aerodynamic and that from Monaco onwards the arrangement must be changed.
The teams and the governing body must dance around quite an intricate line of interpretation on matters such as this. Both the Red Bull RB14 and Force India VJM11 feature aerodynamic strakes as part of their mirror mounts that help condition the airflow behind, but in these cases they are less extreme than the Ferrari interpretation.
It illustrates that for the second consecutive season, Ferrari under the technical directorship of Mattia Binotto is aggressively pushing the boundaries after years of quite conservative design and development.